Slay the Spire 2 Boss Guide: Strategies for Every Major Encounter

The first time Hexaghost killed me, I was genuinely confused. I had 72 HP going into the fight — a great Act 1, solid deck, feeling confident. Then it hit me for 30 damage on turn 2. Then 30 again. Dead by turn 5.

Hexaghost scales its damage to your max HP. The more health you have, the harder it hits. Resting before this fight makes it harder. I had to learn that from a loading screen tip.

StS2 bosses have mechanics like this — things that aren't obvious until they kill you. Here's what I've learned from each one.

Act 1 Bosses

Hexaghost

The gimmick is the HP scaling. Its first big attack, Inferno, deals approximately 40% of your current max HP. At 75 HP you take 30. At 40 HP you take 16. At 20 HP it still deals these massive hits so you can't just cheese it with low health — you need block.

Besides Inferno, Hexaghost fills your deck with Burns and summons burning orbs that deal chip damage. AoE helps clear the orbs. The Burns dilute your draws, so a thin deck or exhaust effects (True Grit, Second Wind) make the fight easier.

Character counters: Ironclad can heal through chip damage with Burning Blood. Silent poison ignores the orb phase. Defect frost orbs block the Inferno hits. Necrobinder wants Souls banked before the fight starts — Reap clears orbs and hits the boss.

Slime Boss

Splits at half HP. The split slimes apply Goop, a new StS2 debuff that increases the cost of the next card by 1. If both splits are alive and both apply Goop, you're paying +2 for your next card — it's crippling.

Strategy: bring AoE. Whirlwind, Cleave, Die Die Die, Electrodynamics. Burst the boss to slightly above half, then use AoE to kill both splits on the same turn. Killing one split and leaving the other alive means eating Goop stacks every turn.

Character counters: Ironclad Whirlwind is ideal. Silent Corpse Explosion doubles as AoE and boss damage. Defect lightning orbs hit random targets — not great for focused split kills. Necrobinder Reap with 15+ Souls clears everything.

The Guardian

Defensive mode for several turns, then offensive burst. During defensive mode it gains block. During offensive it hits for 18-20 with multi-attacks.

Strategy: set up your scaling during defensive mode — play powers, apply debuffs, stack Strength or Focus. Save burst damage for offensive mode. The Guardian isn't the hardest Act 1 boss but it punishes decks that can't block the offensive burst.

Act 2 Bosses

The Collector

Summons two minions that buff the boss when killed. The order matters — you want to kill minions that give the least dangerous buff first, or kill the Collector fast enough that minions don't matter.

AoE still helps for the minions, but single-target burst for the Collector is the real requirement. A 200+ HP boss that buffs itself every turn snowballs fast.

Bronze Automaton

Traps one of your cards in stasis. You have to decide: free the card by dealing enough damage to the Automaton, or let it stay trapped and fight with one less card.

The trapped card is often one of your best — the Automaton targets high-value cards. If it's your win condition piece, fight to free it. If it's a Strike (rare, but it happens), just ignore the stasis.

The Champ

Phase change at half HP. Below 50%, gains massive Strength and uses a heavy multi-hit attack. You have two approaches: burst from 51% to 0% in one turn (requires damage potions and a clean hand setup), or stack enough block to survive the enraged phase and kill over 2-3 turns.

The enrage attack can deal 40-50 damage unblocked. Impervious, Wraith Form, or a Ghost in a Jar potion are clutch.

Act 3 Bosses

The Awakened One

Gains 1 Strength every time you play a Power card. On higher Ascensions, 2 Strength per Power.

This boss single-handedly invalidates power-heavy Defect decks if you don't adjust. Limit yourself to 2-3 powers — your absolute best ones. Everything else stays in hand or gets skipped during deck building.

Phase 2 summons cultists. Kill them quickly — they chip damage and provide ritual buffs. The Awakened One itself has two phases, so plan for a long fight.

Time Eater

Ends your turn after you play 12 cards. Heals a small amount each time it does this.

Shiv Silent and discard-cycling builds get hard countered. If you're running a high-card-count deck, you need to pivot before this fight. Play 5-6 big cards instead of 12 small ones. End your turn at 11 cards intentionally — don't try to sneak in a 12th.

Donu and Deca

Two bosses that buff each other. Donu gives Strength, Deca gives block. The decision is which to kill first.

General rule: kill Donu first. Strength scaling snowballs faster than block scaling, especially on higher Ascensions. But assess your deck — if you have massive single-target damage, kill Deca first to stop the block buffing. If you have scaling AoE, damage both evenly.

Corpse Explosion on Silent is excellent here — killing one transfers poison to the other.

The Heart (Act 4)

Three keys across one run: Ruby (campfire Recall), Emerald (burning elite), Sapphire (skip chest relic). Collect all three before the Act 3 boss and Act 4 opens.

Heart attacks: Beat of Death (1 damage per card played, 2 on A19+), multi-hit (67 damage spread across hits), single massive hit (45+), and Doom stacks (delayed burst damage, new in StS2).

You need one of two things to win: a passive block engine that negates Beat of Death (Feel No Pain, After Image, frost orbs with high Focus), or enough damage to kill before the Doom timer plus Beat of Death chip kills you.

On A20 the Heart has 750 HP and the damage cap is 300 per turn on A19+. Sword Boomerang (4 hits) gets around the cap better than a single Heavy Blade swing.

Post-Boss: Ancient Blessings

After each boss you choose an Ancient blessing. The blessing's penalty is permanent. Some blessings that seemed fine after Act 1 become run-ending by Act 3.

My heuristic: if the penalty affects your sustain or defense, you need to have a plan for that. "No campfire healing" means you need alternative healing — Reaper, Soul Siphon, Meat on the Bone, Eternal Feather. "-20% max HP" means you need to block more consistently because your HP buffer is smaller.

The Ancients system is the biggest strategic shift from StS1 to StS2. In the original, boss relics were mostly upside with a manageable downside. Ancients are genuine tradeoffs. Pick wrong and it'll cost you the run, even with a good deck.