Slay the Spire 2 Act 3 Boss Complete Strategy Guide

Last updated: June 2, 2026 | Reading time: 10 min | by STS2 Guide Team

The Act 3 boss is where most runs die. These bosses have 300-400 HP, deal 30+ damage per hit, and have mechanics that punish unprepared players.

The Awakened One (Hardest Boss)

Phase 1 HP: 300. The Problem: Gains 15 Strength when damaged. If you deal damage too fast, it one-shots you.

Phase 1 Strategy (The Slow Kill): Deal damage in small chunks (15-25 per turn). Dont burst. Kill in 5-8 turns. This is the #1 A20 Boss Killer. Players who deal 80 damage in 2 turns die.

Phase 2: At 50% HP, transitions. Deals 8x3 hits (24 damage) + 1 big hit (30+). Block the multi-hit and kill in 2-3 turns.

Time Eater (Most Annoying Boss)

HP: 380. Key Mechanic: Time Warp every time you play 8+ cards in a turn, he gains 1 Time Warp. At 3 stacks, he takes an extra turn.

Strategy: The "Play Fewer Cards" Approach. Limit yourself to 6-7 cards per turn. Focus on high-value cards. If you have 2 energy and a 0-cost card, dont play it. Let it sit in your hand.

Donu & Deca (The Co-op Boss)

Donu HP: 200. Deca HP: 200. The Problem: They buff each other every turn. If you dont kill one fast, theyll have 30+ buff and one-shot you.

Strategy: Focus Fire. Kill Donu first (his Strength buff is more dangerous). Use AOE to hit both simultaneously. Ideal Deck: AOE damage + high single-target burst.

General Act 3 Boss Preparation

• Arrive at >70% HP. These bosses take 6-10 turns. You need HP to survive.
• Bring AOE. Donu & Deca require it.
• Have block scaling. 15 Block/turn isnt enough. You need 30+ (Metallicize, After Image, Frost Orbs).
• Have damage scaling. 15 damage/turn isnt enough for 400 HP. Need 30+ (Strength, Poison, Shivs, Scrawl).