I previously wrote a strategy guide for solo Serpent here, but that was back when I first started and hadn't played many games yet, and also before I read tips from u/mongooseroar. In light of someone asking for help with HLC here earlier, I figured I'd post an updated strategy guide. I don't have time to go into full depth, and I have yet to "solve" France 6 anyway, but hopefully this will help people.

First off, I'm assuming the Steam version of the game, which currently has Base+B&C+JE+Horizons+Promo 1, but no Promo 2 or NI yet. (And also has GtS replaced by Roiling Bog, and Outpaced and a couple other events retired). Adding additional expansions won't change the overall strategy. The main effect is that a) I won't talk about Scotland or HME since I haven't played them yet, and b) adding NI changes the major power deck, so you're less likely to get any specific major. It also changes the fear, event, and blight card decks, but those have more minor effects.

Abbreviations:

I'll use t1, t2, t3, etc. to refer to turn 1, 2, etc. I'll use pn to refer to when you've removed n presence from your track, e.g. p4 is when you uncover your Any element. By default, this will be the same as the turn count, but ideally you'll be using your innate to place an extra presence most turns as well.

BP = Brandenburg Prussia, HLC = Habsburg Livestock Colony

Abbreviated power names:

Aegis = Elemental Aegis

Absorb = Absorb Essence

Flowing Power = Gift of Flowing Power

Deeps = Gift of the Primordial Deeps

Dream = Dream of the Untouched Land

Twinned Days = Gift of Twinned Days

Spur On = Spur on with Words of Fire

Growth and presence

You pretty much always want to use the fourth growth option to place a presence every turn, so in practice, Serpent's growth functions similarly to A Spread of Rampant Green, where you always place a presence each turn and then pick one of three other options too (Reclaim, Power Card, or Energy, in Serpent's case). There are rare cases where you might do something else, but those should be clear if you end up in that situation (for example, if you literally will lose otherwise, or if you can't threshold Dream otherwise).

In terms of tracks, you always want to get 2 plays ASAP, then the Any element. Your normal track will thus look like this:

p1: Moon p2: 2 plays p3: Fire p4: Any p5-6: Reclaim or Water p7: Earth p8: 4 plays p9: 5 plays or 6 energy

In rare cases, you may want to get 6 energy before 4 plays - you'll know if you need that. Otherwise this is basically fixed.

Action Economy

Solo Serpent has cheap and powerful uniques and innates that it can trigger every turn just by playing its uniques. It normally has no trouble generating lots of energy. Therefore, your biggest constraint is number of plays. In fact, I divide the game into stages based on how many plays you have:

Early game: p1 (1 play)

Mid game: p2-7 (2 plays)

Endgame: p8+ (4-5 plays)

The mid game is further divided into early-mid (p2-3) and late-mid (p4-7) based on whether you have your Any element yet or not.

Serpent really really wants three plays a turn, so it can play all its uniques every turn. However, it is stuck on two plays for most of the game and so has to decide between defence or turbogrowth each turn. Therefore, anything that gets you your third play is really amazing. In particular, you should always take Back Against the Wall unless you'd immediately lose (2 blight cards can be redrawn in solo play, but they're usually good enough that you want to keep them anyway). Likewise, Twinned Days is so insanely good because it gives you a third play each turn.

Innates

Left innate: Turbogrowth

Your left lvl2 innate is extremely strong, and so much of your strategy revolves around trying to trigger it. I refer to triggering the level 2 innate as "turbogrowth".

To hit it, you need 3 Earth and 2 Plant. It may not seem possible to get 3 elements on 2 plays, but solo Serpent actually can hit it as early as t2 with a bit of luck.

T2 Turbogrowth

To do this, you need to play Flowing Power + Deeps + Absorb, and then hit an Earth+Plant minor off of Deeps. Then, use Absorb to get the second Plant and you've hit your threshold.

There are 13 earth+plant minors or 14 including Elemental Boon. However, Gift of Nature's Connection also works because you can just get 2 Plant and then get the last Earth from Absorb. And Twinned Days also works, since you can just repeat everything.

Therefore, there are a total of 16 hits out of 100 minor powers. The odds of hitting 1 out of 4 of these is 50.8%.

However, that's just the average. The exact odds will change slightly depending on what you saw for your 4 draws on the t1 draft. Here are the probabilities depending on how many hits you wasted on turn 1:

0 - 52.4%

1 - 49.9%

2 - 47.3%

3 - 44.7%

4 - 41.9%

Additionally, if you have a Plant+Earth power in hand, then Spur On becomes a hit too, so look one level up in that case. The exact numbers don't matter that much here, since whether you'll go for it or not depends mostly on the board situation. It's not like you'll go "oh it's 40% instead of 50%, better give up".

P4 Turbogrowth

Once you've hit your Any element, turbogrowth is 100% deterministic. Even if you hit four powers with no Plant or Earth at all, you can just not play it to get an extra Earth, and then use Absorb + Any for the Plants.

Therefore, under normal circumstances, t2 and t3 turbogrowth will be a coin flip, while it's guaranteed after that. If you succeed on the first attempt, you'll hit Any on t3 instead.

The biggest issue is that the invaders generally aren't so kind as to let you ignore them in the mid-late game like that. There will be so much pressure that it's hard to take a turn off for turbogrowth later on.

Right innate: City Crusher

Your right innate scales with the number of elements, so there's less of a specific threshold to hit. However, the basic target is 3 Fire + 3 Earth + 2 Moon. This lets you kill a city and push a town (and get 2 fear), which I refer to as "The City Crusher". You can deterministically hit this threshold every turn just by playing your uniques.

Ideally, killing a city and pushing a town is enough to leave a land with only one explorer, saving it from the ravage. Only Sweden and BP are fully vulnerable to the city crusher though. Against England and HLC, you can just kill a town each turn, while Russia still ravages when an explorer is left alone. However, it's still a useful pattern to keep in mind.

Minor powers

As mentioned above, solo Serpent is tightly constrained on plays. Normally, you'll be doing either Absorb+Deeps or Absorb+Aegis every turn in the mid game, so you don't have actions to spare for playing minors. Therefore, you'll typically only be playing a minor on t1, when you use Deeps, and again in the end game when you have 4+ plays it barely matters and you just need elements.

The one exception is the two minors that don't cost you an action.

Twinned Days

The reason Twinned Days is so good is that by repeating Flowing Power, you effectively get three plays per turn (not including Twinned Days and Flowing Power themselves. This means you can play Absorb+Aegis+Deeps every turn). If you get Twinned Days early, it's practically an automatic win (you still can get screwed by a very unlucky event though). Since Twinned Days is basically an autowin button, my strategy guide is written under the assumption that you don't have it. Just keep in mind that in 4% of games, you'll hit t1 Twinned Days and cruise to victory.

Spur On

Spur On is basically a second worse copy of Flowing Power that costs an energy to use. Since it costs an energy and doesn't do anything by itself, you won't just play it all the time. However, it still has its uses.

The biggest use for Spur On is that it lets you threshold majors early that would otherwise be impossible to threshold until the endgame, such as Powerstorm, Irresistible Call, Storm-Swath, Vigor of the Breaking Dawn, Unlock the Gates of Deepest Power, etc. Powerstorm is especially notable because it goes from near-useless in normal circumstances to amazing when you have Spur On, since Spur On both lets you threshold it AND gives you the extra powers to take advantage of the repeat.

To be clear, Spur On is not an autopick like Twinned Days. Again, it does nothing by itself. It's more something you pick if there aren't other good options, since it might come in handy later.

The other 98% of minors

In all other cases, you'll just be playing a minor t1, or when using Deeps. In the case of Deeps, you're mostly picking for the elements (i.e. Plant+Earth) and it's highly situational, so I won't say much. Just look for whichever one seems most useful in the current situation.

That leaves the t1 minor draft.

For your t1 play, you want to do what you can to delay the invaders by stopping a ravage or build. Which powers will work depends a lot on the specific adversary, but generally, you're looking for fast explorer pushes or kills (Treacherous Waterways, Steam Vents, etc.) or things that can kill a town in slow (Call of the Dahan Ways, Rouse the Trees and Stones, etc.)

Failing that, just do what you can (maybe remove a blight if you can and aren't playing HLC). If there are no good options, pick a minor with good elements. If there's an Earth+Plant, choose that in order to turn on Spur On as a t2 turbogrowth hit (see above), otherwise try to pick a power with a Sun element so you can discard/forget it to pay for Missionaries Arrive. Failing that, look for Plant, Air, or Animal elements.

Also pay close attention to Sacred Site requirements. Against Sweden, you can never have an Sacred Site on t1, since you always start in blight. Against other adversaries, you'll start in blight 50% of the time.

Basic strategy

As alluded to above, your midgame strategy will mostly consist of trying to hold back the adversary enough to give you the breathing room to go for turbogrowth and hasten the end game. Sometimes you'll have to go for a major midgame instead, though this is usually a desperation play (keep in mind that most majors also prevent you from hitting your innates).

Your main choice is between Absorb+Aegis for defence or Absorb+Deeps for growth. Ideally, there would be no ravage at all, making the choice easy. More realistically, you should aim to have only one land blight per turn, which is an acceptable cost for some early growth. Using City Crusher can help ensure that this happens on the next turn.

Majors

In the endgame, it doesn't matter much what you get, so I'm evaluating major powers under the assumption that you pick them in the midgame. There are some majors like Bloodwrack Plague or Instruments of Their Own Ruin which are good if thresholded, but which are impossible to actually threshold before the endgame, making them poor picks for an early major.

The Big Three major powers for Serpent are Dream, Indomitable Claim, and Unrelenting Growth, as these three let you place extra presence (Promises of Power and Protection technically does as well, but it is absolute garbage and doesn't count).

Dream is the best major in the game, since it instantly sends you to the endgame if you threshold it (remember you'll also get a third extra presence from your innate), while also holding the adversaries back a bit.

Indomitable Claim is "only" one per turn but also defends. Unrelenting Growth is the worst of the three since it is slow and doesn't defend or remove blight, but it's still good as long as you weren't reaching for a major because you're immediately about to lose.

Apart from the Big Three, major choice is more situational. Some majors that synergize well with Serpent's elements include Forest of Living Obsidian, Hazards Spread Across the Land, and The Land Thrashes In Furious Pain.

General starting plan

First look at the board and decide whether you're going to be defending t2 or not. This depends on the adversary and the board position. BP requires you to defend t2, while HLC and Russia make defending t2 pointless.

Next draft a minor, following above instructions. If you didn't get anything good and you're planning to defend t2, then play Deeps in hopes of hitting something better. Otherwise, you may have to just do nothing t1.

T2 take +4 energy from growth, and then play Absorb+Deeps or Absorb+Aegis. The good news is that your uniques are cheap and Absorb gives you 2 energy every time you play it (and you often get 1 from your innate too), so you can afford to reclaim and do this every turn from then on.

Events

One event (Years of Little Rain) gives you a free presence if you have Dahan in your land. Therefore, from t2 on, try to always keep a presence in a land with a Dahan if you don't have any particular need to place your presence elsewhere. That way, you'll be able to benefit just in case it comes up.

Adversary specific strategy:

In rough order from easiest to hardest:

Sweden

Sweden is close to vanilla, and the best demonstration of the "turbogrowth while one land blights, use city crusher to clear out a land for next turn" pattern. 75% of games are very easy. Sweden's only real threat is if the initial explore is in land 8 (the mining outpost), where they threaten 5 blight on turn 2. This is a much tougher situation, and you'll often lose these games. You basically have to hope you get lucky on minors, or the events treat you well and you pull through anyway.

Tip: Whenever you have the chance to move Dahan (e.g. from events), try to group them up to avoid the stage 2 escalation. Moving them into lands that already stage 2 explored also works.

Tip: Instead of the usual start, considering doing +4 energy and Flowing Power + Deeps on t1, and then reclaim for turbogrowth on t2. This guarantees a t1 town push and gives you a slight chance of being able to take out a city t1 instead.

BP

The other "vanilla" adversary. T2 Aegis is effectively mandatory, but you have a great chance to go for turbogrowth on t3 and t4 while they're "only" ravaging in two lands each turn (especially if you cleared one out before with your innate). Watch out for overlapping explores though, as that makes things tougher. Often you'll end up with two explorers in a land and it still ravages even with City Crusher.

England

England isn't exactly vanilla - you can only kill a town each turn with your innate (unless combined with other damage), and you have to constantly watch out for the loss condition. Keep an eye on the number of towns and cities in each land and try to keep the max at only 4 or 5.

The good news is that England is slow enough that it's relatively easy anyway. You don't even have to go for turbogrowth, normal Aegis stalling works well too. You will probably have to go for a major or two in the midgame to help clear out the building spam though. Here, you're mostly just looking for high damage nukes like Cleansing Flood or Fire and Flood. Paralyzing Fright is also good for stopping builds and giving you fear.

Tip: Remember that the extra build happens before ravage, and take that into account when calculating defends.

HLC

Now come the adversaries with crazy rules.

• Waves of Immigration happens on t4 after the explore. Stage 2 escalations are on turns 2, 3, 4, and 5.
• HLC's big thing is towns and blight. You're in an all out war against town spam here. Note that even an empty land will still gather a town after the build. However, it's still valuable to prevent builds in order to limit the number of towns being added. This goes double for inland lands, which build double.
• Isolate effects are especially good against HLC, since they stop the town gather on the build step and the +2 damage on ravage. Set Them on an Ever-Twisting Trail is one of the few minors that can actually solve things t1.
• Against HLC, you actively want to take two blight on t2 in order to limit the escalation and reduce town health. This is the perfect time to go for a t2 turbogrowth.
• Drought and Scour the Land are actually good t1 plays, unlike every other adversary, where you want to avoid adding blight at all costs.
• You can kill a town with your innate every turn by doing 2 damage and then pushing it into a land with blight.
• You're probably going to have to switch to defending in the midgame, after letting so much blight through early. Try to be in a good position when this happens to maximize your chances.
• Dream on a blighted land with a town lets you remove a town (and explorer) and blight at the same time, even though removing the blight should increase the town's health. I'm not sure if this is how it is supposed to work or a bug in the app, but it's worth noting.

Example game against HLC6

Russia

• Stage 2 escalations are on turns 3, 5, 7, and 9.
• Russia's big thing is explorers. You're in an all out war against explorers here. This is much tougher than HLC because dealing with explorers is what Serpent is worst at (they're immune to your innate for instance). And Russia makes explorers especially tough and dangerous for good measure.
• Against Russia, explorers are more dangerous than towns. Prioritize removing explorers when you can. For example, with Call of the Dahan Ways, you should usually take out an explorer, not a town.
• Try to avoid grouping up explorers as much as possible, and in particular avoid hitting three in one land. This will inevitably happen eventually, but delaying it helps a lot. The one exception is if you have a powerful major that's going to deal with the land, in which case of course, group them up if applicable.
• Typically, you'll kill explorers by having Dahan deal damage to 2-3 explorers, with one escaping and the others dying. Killing an explorer is better than killing the town here. Same applies if you have 2+ damage from other sources (like majors).
• Isolate effects are especially good against Russia. An explorer in an isolated land can't escape, it just dies.
• Learn how the explorer escape rule works - it's pretty counterintuitive. It's one per action that survive. What exactly counts as an action is unfortunately fairly confusing.
• Pressure for Fast Profit effectively forces you to allow one land to blight in the ravage every turn. You really do not want Pressure for Fast Profit to take effect. That extra explorer and town is a lot more devastating than it might look on paper.
• The main exceptions to allowing blight are a) you have Back Against the Wall and are forced to defend everything, b) you already have good majors and can defend everything or c) you have a nuke in play that can wipe out everything in the slow phase anyway (usually this requires a major, but Poisoned Dew also works.)
• The good news is that being forced to let a land blight every turn means you have plenty of opportunity to go for turbogrowth. The bad news is that the blight card will flip and pretty quickly too.
• Instead of trying to protect every land, your big challenge is to protect all but one land. If two lands are going to have a bad ravage at once, you may have to defend one, which is pretty painful since you take the blight and you don't even get a turbogrowth in exchange!.
• Remember, you can deliberately position your Aegis so it only protects one of the lands that are going to ravage. Or all but one, since after a few turns you'll usually have lots of lands ravaging each turn.
• Sometimes you'll get screwed by a random event or fear card preventing the ravage. "Invaders build instead of ravage" effects are especially punishing because you get Pressure for Fast Profit and an extra build.
• Blight removal is especially good against Russia. You're forced to allow at least one blight every turn, but there are no penalties for picking it back up again unlike with HLC. When hitting random minors off of Deeps, be sure to look for opportunities to remove blight (as a bonus, these are usually the Earth+Plant powers anyway).
• You're going to eventually need majors to deal with the explorer hordes. What you're looking for are majors that are fast and do a little damage, not necessary raw damage output. Thickets Erupt is more useful than Pillar of Living Flame here. Melt Earth into Quicksand is especially good since you can easily threshold it and it isolates too. And Sleep and Never Waken is just absolutely godlike against Russia.
• Avoid unnecessary fear until you've flipped the first entrench card. Remember, elemental thresholds are optional. When using your right innate, be sure to cancel the moon part to avoid the extra 2 fear. Likewise, with Visions of Fiery Doom, be sure to cancel the threshold. And avoid powers that just create fear without doing anything useful. Delaying the first Entrench by a turn or two may not seem like much, but it makes a HUGE difference in your chances of winning.
• On Entrench turns, lands with 3+ explorers will ravage twice, so watch out for that. If you leave one undefended, it will blight, and then blight again and cascade!
• If you get Pent-Up Calamity early, removing both beasts (if they haven't killed any yet) will completely shut off the stage 2 escalations and alternate loss condition. Remember, you don't have to put the tokens back with the threshold if you don't want to!

France

Lol, good luck.

France is the only lvl6 I haven't found a good strategy for yet. I have won, but it's only like one game in three. The problem is that France wins on the build step, and very quickly too. You're basically in an all out race to stop builds here. I think the best bet is to just pull a major really early (e.g. t3, possibly even t2) and hope you get a good one.

What you're looking for here is fast damage, ideally in multiple lands, rather than large numbers. Tigers Hunting is decent here (watch out for blight though) as is Sleep and Never Waken. Also good are build skippers like Venomous Spiders or Celestial Brilliance. If you can move all the explorers into one land and just skip the build there, that helps a lot.

Moving explorers and towns around can also help to limit the damage from builds. Flow like Water, Reach like Air is a surprisingly good power against France. It may seem like it does nothing at first glance, but moving the explorers and towns around can save you from a terrible build.

Also remember that ravage happens before build. When counting towns to see if you're dead yet each turn, be sure to take into account that the Dahan can counterattack and kill some towns before the build happens. You just have to pray that you don't get one of the "skip ravage/counterattack" events.

Keeping your Dahan alive is especially useful against France. Also remember that the event on turn 4 will kill a town in a land with 2+ Dahan. Try to make sure there is one of those so you benefit.