Slay the Spire 2 Boss Guide: How to Beat Every Boss (Acts 1-4)
Table of Contents
1. Act 1 Bosses 2. Act 2 Bosses 3. Act 3 Bosses 4. Act 4 — The Corrupted Heart 5. How to Prepare for Each Act Boss"How do I beat [Boss X]?" is the single most-searched question in Slay the Spire 2. This guide covers every boss across all four acts — their attack patterns, what cards counter them, and exactly how to prepare for each one before the fight even starts.
1. Act 1 Bosses
You'll face one of three bosses at floor 16. The boss is revealed on the map, so you can prepare once you know which one you're fighting.
🐉 Slime Boss
HP: 140. Special: Splits into 2 Medium Slimes at half HP (70), each with 36-40 HP.
Attack Pattern: Prepare (debuff) → Slam: 35-38 dmg → cycle. After first Slam, pattern accelerates. At half HP, boss splits immediately and the fight becomes a 2-enemy battle.
How to Beat:
- Control the Split: The most important concept. When Slime Boss is at 75-80 HP, spend a turn doing nothing except block. Then burst him as far below 70 HP as possible — ideally to 40-50 in one turn. This leaves the Medium Slimes with low starting HP.
- Bring AoE: Whirlwind (Ironclad), Dagger Spray (Silent), Sweeping Beam (Defect), Corpse Explosion (any). AoE lets you kill both Medium Slimes simultaneously.
- Watch for the Slam: It hits for 35-38. If you can't full-block, use Weak (Shockwave, Neutralize) to reduce it to 26-28.
- Save your potions: A Fire Potion or Explosive Potion makes the split burst trivial.
Best Cards: Whirlwind, Cleave, Immolate, Dagger Spray, Sweeping Beam. Any AoE is invaluable here.
🛡️ The Guardian
HP: 240. Special: Alternates between Defense Mode (takes 0 damage in bronze form, gains block) and Attack Mode (vulnerable). Enters Defense Mode after taking 30-35 damage.
Attack Pattern: Charging Up: gain 9 block → Fierce Bash: 32 dmg → Vent Steam: debuff -1 Dex/Str → Defensive Stance: +20 block → repeat. Pattern fixed, not random.
How to Beat:
- Exploit Defense Mode turns: When Guardian curls up, he does NOTHING for 1-2 turns. Use these turns to play Powers, set up scaling (Demon Form, Noxious Fumes, Defragment), and heal if needed.
- Pace your damage: Don't burst him — you'll trigger Defense Mode faster. Deal exactly enough damage per cycle to break through his block, then set up on the Defense turns.
- Block the Fierce Bash: It always comes after Charging Up. You get exactly 1 turn warning. Have 32+ block ready or use Weak.
- Vent Steam is temporary: The -Dex/-Str debuff fades after combat. Don't stress about it, just block through it.
Best Cards: Powers of any kind (free setup turns), Spot Weakness, Footwork, Defragment. Scaling cards that you can set up during Defense Mode turns.
👻 Hexaghost
HP: 250. Special: Turn 2 attack damage scales with your current HP — higher HP = bigger hit.
Attack Pattern: Turn 1: 2×6 damage → Turn 2: 2×6 OR big single hit (scales with HP) → Turn 3: 3 Burns added to discard → Turn 4+: multi-hit + Burn cycling
How to Beat:
- Don't heal before the fight. Hexaghost's Turn 2 nuke is based on your HP. At full health (70 HP), it hits for ~36. At half HP (35), it hits for ~18. Counter-intuitive but true: entering the fight injured actually reduces the damage you take.
- Bring scaling damage. Hexaghost has 250 HP and no damage cap. A Strength build or Poison build will out-scale him. The fight is a DPS race.
- Manage the Burns. He shuffles 3 Burns into your discard pile. These thin your effective draw — you'll see fewer of your good cards. Medical Kit trivializes this. Without it, make sure your key cards are already in hand before Turn 3.
- His multi-hits are individually weak. 2×6 damage means 12 total. A single Shrug It Off blocks the entire attack. Don't over-block.
Best Cards: Demon Form, Noxious Fumes, Defragment. Any long-term scaling. Disarm reduces multi-hit damage to zero.
2. Act 2 Bosses
Act 2 bosses are widely considered the hardest fights in the game. You'll face one at floor 33.
🏆 The Collector
HP: 280. Special: Summons 2 Torch Head minions (38-40 HP each). Main threat comes from minions, not the boss.
Attack Pattern: Turn 1: Summon Torches → Turn 2: Buff (all gain +3 Str) → Turn 3: Attack (16-18) → cycle. Torch Heads attack for 7-9 each turn.
How to Beat:
- Kill Torch Heads IMMEDIATELY. On turn 1, there are 3 enemies: Collector (280 HP) + 2 Torches (38-40 HP each). If both Torches survive to Turn 2, the Collector buffs them. A 7-damage hit becomes a 10-damage hit. Kill at least one before Turn 2.
- Turn 1 AoE is premium. Whirlwind, Immolate, Dagger Spray, Electrodynamics — anything that hits multiple enemies on turn 1 is S-Tier here.
- After Torches die, the fight is free. Collector alone deals 16-18 damage every 3 turns. That's trivially blockable. The challenge is killing Torches before they scale.
- Weak on Collector is fine but not essential. His individual damage is low. Focus on Weak for the Torches in the first 2 turns.
Best Cards: Any AoE. Disarm or Malaise on Torches. Burst damage to kill minions within 2 turns.
🤖 Automaton
HP: 300. Special: Fires Hyperbeam every 6 turns. Gains Strength each cycle.
Attack Pattern: Turn 1: spawn 2 Orbs → Turn 2: 12-15 dmg + 1-2 Weak → Turn 3-5: 16-22 dmg attacks → Turn 6: HYPERBEAM (45-50 dmg) → repeat with +3 Strength.
How to Beat:
- This is a DPS race. Automaton gains +3 Strength every 6 turns. If the fight lasts 3 cycles (18 turns), he'll have +9 Strength — his basic attacks hit for 30+ and Hyperbeam hits for 60+. You must kill him within 10-12 turns.
- Kill the Orbs early. They deal 6-8 damage each and apply debuffs. They're weak (15-20 HP) — one strong attack kills them. Don't let them accumulate.
- The Hyperbeam turn: BLOCK EVERYTHING. On turn 6 (and turn 12), Automaton does NOTHING else. No minion attacks, no debuffs. Spend all your energy on block. Impervious, Wraith Form, Buffer — anything that negates a single massive hit.
- Intangible breaks the fight. Wraith Form or Incense Burner timed to the Hyperbeam turns makes you take 1 damage instead of 50.
Best Cards: Wraith Form, Impervious, Buffer, Corruption + heavy block. Scaling damage — you need to kill before turn 18.
🧛 Champion
HP: 420. Special: Invincible at half HP; transforms into a stronger form. Clears all debuffs on transform.
Attack Pattern (Phase 1): 17-20 dmg → +Metallicize → 20-25 dmg + Frail → cycle. Pattern (Phase 2): 30-35 dmg → Apply Vulnerable → 45-50 dmg → cycle.
How to Beat:
- Phase 1: Build up. The Champion's Phase 1 damage (17-25) is manageable. Use this phase to play ALL your Powers. Demon Form, Noxious Fumes, Defragment, Footwork — get everything online before you push him below 210 HP.
- Phase 1 stall until ready. There's no penalty for dragging out Phase 1. Play block cards. Don't attack more than necessary. Once all Powers are played and you have a full hand of attacks, unleash.
- Phase 2 is brutal. 30-50 damage per turn. You need heavy burst — the Champion must die within 3-4 turns of Phase 2. If you have 200+ damage potential (Strength-scaled Heavy Blade, stacked Poison, Dark Orb recursion), pop it all.
- Don't waste debuffs in Phase 1. All debuffs (Weak, Vulnerable, Poison) are cleared on transform. Apply them only after Phase 2 starts.
Best Cards: Scaling Powers that accumulate value passively (Demon Form, Noxious Fumes, Creative AI). Burst damage in Phase 2.
3. Act 3 Bosses
Act 3 bosses test whether your deck has a coherent win condition. You'll face one at floor 50.
⏳ Time Eater
HP: 456. Special: After you play 12 cards, Time Eater ends your turn, gains +2 Strength, and cleanses debuffs. The card counter resets each turn.
Attack Pattern: 7-9×3 dmg + Draw Reduction → 26-30 dmg → 32-36 dmg + Vulnerable → Ripple: No attack, shuffles Slimed → cycle.
How to Beat:
- Plan your 12-card turns. This is the single most important concept in all of STS2 boss strategy. Before you play anything, count: "I have 12 cards this turn. What are the most impactful 12 plays?" Cards 1-10 should be block and scaling. Card 11 should be your biggest debuff (Weak, Vulnerable). Card 12 should be your heaviest attack — make Time Eater earn that turn-end.
- High-impact cards over lots of small ones. Bludgeon (one card, 42 damage) is better than 10 Shivs. Heavy Blade at 15 Strength (one card, 60 damage) is perfect. Avoid Shiv decks and infinite combos unless you can kill within 3-4 turns.
- Ignore the Slimed. Time Eater shuffles Slimed into your discard. Unless you have Medical Kit, just accept the smaller effective draw. Don't spend energy exhausting them.
- Don't panic-burst at 12. Time Eater ends your turn — you don't take damage for hitting 12 cards. The penalty is the +Strength. At +4 Strength (after 2 cycles), his attacks hurt significantly. Kill him before he reaches +6 to +8.
Best Cards: Bludgeon, Heavy Blade, Demon Form, Wraith Form, Malaise (Strength reduction!). Avoid: shiv-based builds, infinite combos.
🐦 Awakened One
HP: 320 (Phase 1: 320, Phase 2: 320 — revives at full HP). Special: Gains +1 Strength every time you play a Power card in Phase 1. Transforms at 0 HP.
Attack Pattern (Phase 1): 8-10×3 or 16-20 → Soul Strike: 6×4 → cycle. Phase 2: 20-24 + Void to discard → Sludge: 18-22 → Tackle: 10×3 → Dark Echo: 40 → cycle.
How to Beat:
- Control which Powers you play in Phase 1. Every Power gives him +1 Strength. Play only your MOST ESSENTIAL Powers — usually 1-2 per run. Corruption? Yes. Demon Form? Yes. Feel No Pain AND Dark Embrace? Play one, save one for Phase 2. Don't play Niche Powers (Juggernaut, Evolve) until Phase 2.
- Phase 2 doesn't scale from Powers. Once he transforms, play ALL remaining Powers freely. The +Strength mechanic is gone. If you held back Dark Embrace, Second Wind, and Inflame, Phase 2 becomes a victory lap.
- The 40-damage Dark Echo is telegraphed. It always comes after the Tackle turn. You get 1 full turn to prepare block. If you can block 40, you win. If not, use Intangible.
- Strength reduction is excellent. Disarm in Phase 1 reduces his multi-hits to near-zero. Malaise (Silent) can reset his strength entirely.
Best Cards: Disarm, Malaise, Wraith Form. Limit Power plays in Phase 1 to 2 maximum.
🔮 Donu & Deca
HP: Donu: 250, Deca: 250. Special: Two bosses fought simultaneously. Donu buffs both (+3 Strength to all). Deca applies Block and Dazes. When one dies, the survivor goes "enraged" (+5 Strength).
Attack Pattern: Donu: Buff (+3 Str all) → 12-16×2 → cycle. Deca: 16 Block + 2 Dazed → 10-14×2 → cycle.
How to Beat:
- Focus Donu first. Donu is the damage scaler. Every buff cycle adds +3 Strength to both Donu and Deca. After 4 cycles, their basic attacks hit for 22-28 ×2. Kill Donu within 4-5 turns or the fight spirals.
- Block Deca's Dazes. Deca shuffles 2 Dazed into your discard every other turn. These accumulate and thin your effective draw. Without Medical Kit, by turn 6 you'll draw 3 Dazed + 2 real cards. Kill Donu faster, or have a plan for the status cards (Evolve, Feel No Pain + Dark Embrace).
- AoE is incredible here. You're hitting two 250-HP targets. Whirlwind, Immolate, Electrodynamics — anything that damages both simultaneously cuts the fight time in half.
- Kill them simultaneously OR with a 1-turn gap. The survivor enrages. If you leave one at 20 HP for 3 turns, you're eating +15 Strength boosted attacks. The ideal sequence: kill Donu, kill Deca on the next turn.
Best Cards: AoE damage, Strength reduction (Disarm on Donu), Medical Kit, Evolve, Feel No Pain + Dark Embrace to handle Dazed.
4. Act 4 — The Corrupted Heart
The Heart is the true final boss of Slay the Spire 2 (available from Ascension 1+). You must collect 3 keys during your run to access Act 4.
❤️ The Corrupted Heart
HP: 750. Special: Invincible (caps damage at 300 per turn). Beat of Death: deals 1-2 damage every time you play a card. Gains 50% Strength per cycle.
Attack Pattern: Turn 1: Debuff (-1 Draw, -1 Energy, Statuses) → Turn 2: 45-67 dmg multi-hit → Turn 3: 40-45 single hit → cycle. Multi-hit applies 1-2 Frail/Weak/Vulnerable.
How to Beat:
- Turn 1 is brutal. You lose 1 card draw and 1 energy permanently, plus 2-3 status cards are added to your deck. This is the single hardest opening turn in the game. You need card draw on turn 1. Offering, Battle Trance, Adrenaline, Skim. If you start with 3 cards (5 base - 1 debuff - 1 from statuses), you lose. If you start with 8+ (through draw), you can stabilize.
- Beat of Death punishes card spam. Playing 10 cards = 10-20 damage taken. Shiv decks, infinite combos, and Corruption+Dead Branch spam need After Image, Tough Bandages, or Orichalcum to offset the damage. 20-Shiv turns require 20 Block from After Image just to break even.
- You need TWO win conditions: damage cap + survival. The 300 damage cap means you can't one-shot the Heart. You need to survive through 3 rounds minimum (300 + 300 + 150 = 750 damage over 3 turns). That means blocking 45-67 damage turns while outputting 300 damage per turn. Heavy scaling (Strength, Focus, Poison) plus robust block is mandatory.
- The multi-hit turn (Turn 2): BLOCK EVERYTHING. 45-67 damage randomly split across multiple hits. Each hit reduces your block and bleeds through. Impervious, Wraith Form, Buffer are premium. Weak (Disarm, Malaise, Piercing Wail) reduces it to 30-44.
Absolute Best Cards for The Heart: Wraith Form (Silent), Buffer (Defect), Impervious + Corruption (Ironclad), Malaise (Silent), After Image (Silent, for Beat of Death), Feel No Pain + Dark Embrace (Ironclad, exhaust statuses for block/draw).
5. How to Prepare for Each Act Boss
| Act | What You Need | Key Cards | Potion Strategy |
|---|---|---|---|
| Act 1 | Front-loaded damage, 1 AoE card, 25+ max HP | Carnage, Whirlwind, Dagger Spray, Sweeping Beam | Attack Potion or Fire Potion to burst the split/Slam turn |
| Act 2 | Scaling damage, AoE, 40+ block capacity per turn | Demon Form, Noxious Fumes, Disarm, Wraith Form | Block Potion or Ghost in a Jar for Hyperbeam turn |
| Act 3 | Finished deck, strong win condition, block engine | High-impact attacks, reliable scaling, Energy relics | Fairy in a Bottle or Ghost in a Jar as safety net |
| Act 4 | Card draw (turn 1), burst damage (300 cap), block engine | Offering, Adrenaline, Corruption, Wraith Form, Buffer | Ghost in a Jar ×2 is the optimal setup if you can find them |
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