Boss Guide#

Slay the Spire draws one boss at random from a fixed pool at the end of every act: Act 1 bosses appear on floor 16, Act 2 bosses on floor 33. Data on this page reflects an October 2026 web verification, using only the official wiki’s fixed HP values (base / Ascension 9+), never ranges.

Boss Summary Table#

Act 1 Bosses#

Slime Boss#

The split always triggers — there is no “clean the slimes and it won’t split” mechanic. Both large slimes inherit the boss’s remaining HP at the split, so the lower you push the boss before it splits, the easier the cleanup. Plan your burst rhythm ahead: stack block the turn before the split, then focus down the wounded halves afterward.

The Guardian#

The Guardian is about managing the offense-defense rhythm: it mitigates in defensive mode, and your damage windows open in offensive mode. The key is pacing damage around the threshold — don’t waste high-cost output right before the shift, and don’t keep pouring resources into it once it goes defensive.

Hexaghost#

This page does not compile its turn-by-turn patterns — follow the in-game intent icons and the official wiki. The general approach matches other Act 1 bosses: by the end of Act 1 your deck should already have a stable damage-and-block loop.

Act 2 Bosses#

Bronze Automaton#

Multi-target handling is the fundamentals here: the parts change the fight’s rhythm while alive, and whether to clear them first or focus the body depends on your deck’s AoE capacity. Stack block in advance for its heavy-attack turns.

The Collector#

The Collector summons Torch Heads to assist. Summons stretch the fight; whether to prioritize them depends on your deck’s ratio of single-target burst to AoE.

The Champ#

This page does not compile its turn-by-turn patterns — follow intent icons and the official wiki. Act 2 boss HP steps up across the board; make sure your block loop is complete before entering Act 2.

Act 3 Bosses#

The Awakened One#

The Awakened One is the classic Power-card punisher: the more Powers your deck runs, the more Strength you feed it. Either tighten your Power usage or build around Skills and Attacks. Its two-health-bar design means burst resources must be budgeted across two fights.

Donu and Deca#

The twin fight tests AoE and focus order: decks with AoE can grind both down in parallel, while single-target decks should burst one down decisively to avoid both reaching high-threat states at once.

Time Eater#

Time Eater punishes card count: high-frequency small-card decks get hard-countered, while low-frequency heavy-hitting lines are the answer. Its half-HP Haste means debuff tools should either be fully stacked before it hits half HP — wasting Haste’s cleanse timing — or saved until after Haste resolves.

Act 4: The Hidden Act “The Ending”#

Entering Act 4 requires collecting all three keys (see Secrets). Inside, you first face the elite duo Spire Shield and Spire Spear, then the final boss.

Spire Shield and Spire Spear#

Act 4’s elite duo fight — the final exam before the Heart. This page does not compile their turn-by-turn details; follow intent icons and the official wiki.

Corrupt Heart#

The Heart sets two exams at once — card count and per-hit damage ceiling: Beat of Death taxes every card you play, and the invincibility cap discounts high-frequency small hits. The answer is raising per-card damage quality, controlling total cards played, and bringing enough block and healing resources. The final boss’s mechanics and the three keys are detailed in Secrets.

High-Difficulty Note: Ascension 20#

At Ascension 20, the end of Act 3 makes you fight two bosses back-to-back (two consecutive draws from the Act 3 pool). Your build can’t be tuned for a single boss — AoE, single-target burst, and sustain must all be online.

Universal Boss-Fight Principles (Editor’s Experience, Not Official Data)#