Slay the Spire 2 Boss Killers: Walkthrough, Builds & A20 Strategies

The first time I hit the Act 2 boss in Slay the Spire 2, I had what I thought was a solid Ironclad deck. Good strength, a couple upgraded attacks, 40-something HP. Died on turn 4. Didn't even get it below half.

StS2 bosses aren't like the original. The attack patterns are meaner, the HP pools are bigger, and the new Ancients system means your boss reward is a blessing choice instead of a guaranteed power spike. You can't just stumble into victory with a decent deck anymore. You need a plan.

The New Boss Reality

Mega Crit changed how acts flow. Each act now has two possible versions — you might face a different boss lineup run to run. That means you can't memorize one fight pattern and coast. Act 1 could be Hexaghost or Slime Boss with new mechanics. Act 2 could throw the Collector with an extra minion or a completely different fight.

The Ancients system replaces boss relic drops. Instead of "here's your Cursed Key, enjoy," you get offered three blessings from different Ancients — entities from the game's Timeline lore. Each blessing has a drawback. One might give you +1 energy per turn but reduce your max HP by 20%. Another could upgrade all your cards but prevent healing at campfires. You're choosing which downside you can live with, and that choice changes your entire next act's strategy.

Act 1: Survival Means Front-Loaded Damage

The first boss is a gear check. If you spent Act 1 grabbing scaling cards and setup powers, you're dead. Hexaghost scales its damage based on your max HP — sounds scary but it means the fight is predictable. If you come in with 70 HP it hits hard. If you limp in with 30 it hits soft. Weirdly, resting before this fight can make it harder.

Slime Boss got reworked. It still splits at half HP but the split slimes now apply Goop — a new debuff that increases the cost of the next card you play by 1. Nasty if you're running a low-energy deck. Bring AoE and kill both splits on the same turn if you can.

Best Act 1 picks regardless of character: anything dealing 12+ damage for 1 energy. Carnage, Hemokinesis, Glass Knife, Ball Lightning. Pick two or three of these before your first elite or Gremlin Nob will end your whole career.

Act 2: AoE is Not Optional

The Act 2 boss gauntlet is where most runs die. Collector still summons minions, but the minions now buff the boss when they die — so killing them fast is still right but you need to watch the order. Bronze Automaton traps a card in stasis and you have to decide whether the trapped card is worth freeing.

AoE is mandatory by Act 2. Triple slavers, triple cultists, the birds — hallways will shred you without it. Whirlwind, Die Die Die, Electrodynamics. Doesn't have to be fancy, but you need at least one AoE card by floor 2 of Act 2.

For Necrobinder players, Act 2 is where the character wakes up. By now you've got 20-30 Souls from Act 1, and Reap starts clearing rooms. But keep Soul Siphon handy — the sustain matters when you're taking chip damage in multi-enemy fights.

Regent has a completely different approach to Act 2. Quests give you mid-act power spikes — completing objectives like "play 10 attacks in one turn" or "block 40 damage in a single combat" grants permanent bonuses. Plan your pathing around Quest completion, not just campfires.

Act 3: Scaling or Bust

By Act 3 you need a win condition. Not "I have good cards," but a specific repeatable combo that ends fights. Ironclad exhaust engine with Corruption, Dark Embrace, and Feel No Pain. Silent discard loop with Tactician and Reflex. Defect focus stacking. Necrobinder Soul Bomb. Regent completed Quests into empowered finishers.

The Awakened One punishes power-heavy decks by gaining Strength. Time Eater punishes card spam. Donu and Deca scale each other. You don't get to pick which one you face, so your deck needs answers to all three. That sounds daunting but it really means: have block that doesn't rely on playing many cards, and have damage that doesn't require setting up 4 powers first.

The Heart

Still there. Still requires three keys across one run — Ruby from a campfire using Recall, Emerald from a burning elite, Sapphire from skipping a chest relic. Collect all three and Act 4 opens.

The Heart in StS2 has the Beat of Death (damage per card played) but it also applies Doom stacks — a new mechanic that deals delayed burst damage after a countdown. It's like a bomb on a timer. You need to kill the Heart before the Doom counter hits zero, or you need enough block to eat the hit.

My cleanest Heart kill: Ironclad exhaust engine. Corruption into Dark Embrace into Feel No Pain, then just cycled skills for infinite block while Body Slam did the work. Beat of Death wasn't an issue because I was gaining 3 block per card played. Didn't even need to think about the Doom timer — fight was over by turn 6.

With over 570 cards and 299 relics in the game, there's no shortage of possible builds. But the fundamental rule hasn't changed from the original: have a plan, don't draft random cards, and know what your deck's win condition is before you fight the Act 3 boss. The game is early access — balance will shift — but those fundamentals are permanent.