Slay the Spire 2 Act 3 Strategy: Prep Your Deck Before the Final Bosses

The Last Gate Is a Different Test

Most runs that die do it in Act 3. By the time you reach Glory, your deck is locked in. There’s no time to patch a hole you ignored back in the Hive. The boss either matches up well against what you built, or it doesn’t, and you find out the hard way.

The good news is that Act 3 prep is mostly about decisions you make earlier. If you treat Act 2 drafting like Act 3 already started, you walk into the final fight ready. Here’s how to do that after the v0.107.1 update changed the boss lineup.

Know Who You’re Facing

The single most important habit: scroll to the top of the map the moment you enter Act 3. The boss icon tells you who’s waiting. Each of the three bosses wants a different answer, so this one glance decides how you spend your card rewards, your gold, and your rest sites for the whole act.

The current Act 3 Glory pool is Aeonglass, Queen, and Test Subject. Doormaker is gone. The Major Update 2 patch on June 19, 2026 pulled it out entirely and dropped in Aeonglass as the replacement. If you’ve been gone a while and you’re prepping for Doormaker’s old rule-warping tricks, stop. That fight no longer exists. Our Major Update 2 patch notes covers the full swap.

Aeonglass: Build to Race

Aeonglass attacks your clock. It feeds you Wither status cards that stick in your hand on Retain and chip you for damage every turn they sit there, and it keeps adding more as the fight drags. The longer you grind, the worse it gets. Players on the subreddit consistently call it harder than Doormaker, and it has the highest front-load damage check of any current Act 3 boss.

What your deck needs:

• Scaling offense that kills fast. Aim to end the fight around turn 10 or 11. A wide, do-everything deck that never closes loses here.
• Card removal from hand. Discard and exhaust effects clear Withers before they tick. This is your pressure-release valve.
• Artifact or a way to eat the opening. That big front-load means you want either Artifact charges or solid early Block so the first heavy turn doesn’t gut you.

Don’t dump your whole hand on the turns Aeonglass applies Strength and Dexterity debuffs. Read the intent, hold back, and spend your big plays on clean turns. The full fight breakdown lives in our Aeonglass boss guide.

Queen: Survive the Opening, Then Race

Queen fights alongside a Torch Head Amalgam minion and hits like a freight train. Her opening is brutal and fixed: Puppet Strings applies Chains of Binding, then You’re Mine slaps you with heavy stacks of Frail, Weak, and Vulnerable. That Vulnerable sticks, so every hit she lands afterward is amplified.

Chains of Binding is the mechanic to respect. It Afflicts the first few cards you draw each turn with Bound, and you can only play one Bound card per turn. So your hand quality drops right when you need it most.

What your deck needs:

• A way to function through Bound. Card draw helps you dig past Bound cards. Cheap, high-impact plays let your one playable Bound card still matter.
• Defense for the enrage. Killing the Torch Head Amalgam flips Queen into an aggressive loop of multi-hit attacks, a big Execution, and stacking Strength. You want Block built before the minion dies, not scrambled together after.
• Strength of your own. Queen is a fight where a Strength-based deck shines because raw damage punches through her HP before the enrage spiral catches up.

Test Subject: Bring Raw Power

Test Subject is the lowest-HP boss in the pool and rewards decks that simply hit hard. The community read is blunt: any deck with enough raw power beats it. It copies and uses elite abilities and, in its later phase at high ascension, can start spamming multi-hit attacks, so you don’t want the fight to run long.

What your deck needs:

• Damage density. Most strong runs kill Act 3 bosses between turn 7 and 10. Test Subject especially folds to a deck that front-loads damage.
• Strength-down or defensive relics. Reducing enemy Strength and keeping a Block floor smooths out the phase-2 attack spam.
• A clean offensive engine. This is the boss that punishes a deck with no clear win condition. If you’ve been coasting on decent commons, Test Subject exposes it.

Drafting Rules That Cover All Three

You won’t always get to scout before your deck is shaped, so build to a general standard that beats the hardest option.

Keep your deck thin. A tight deck draws its payoff faster, which matters when every Act 3 boss rewards closing the fight quickly. Remove Strikes and Defends at every shop and event you can afford. Our upgrade priority guide helps you spend those rest sites well.

Prioritize relics over extra cards. A relic gives value every combat without needing to be drawn. Energy relics especially translate directly into more plays per turn, which means faster kills. Take 2 to 3 elites per act when your deck can handle them, since elites are your relic source. See our relic synergy guide for the combos worth chasing.

Build a win condition, not a pile of good cards. All three bosses punish a deck with no plan. Decide what your deck does to deal scaling damage by mid-Act 2 and draft toward that. The deck archetypes guide lays out the shells that actually close runs.

Hold a defensive floor. Even race decks need to survive the opening swing. A couple of strong Block cards or a defensive relic keeps a bad first turn from ending the run before your engine comes online.

The Mindset

Act 3 isn’t where you build your deck. It’s where you find out if you built it right. Scout the boss, know what each one wants, and shape your earlier choices around the fight you’ll actually face. Do that and the final gate stops feeling like a coin flip. If you want the deeper boss-by-boss reads, the Act 3 boss guide and our boss strategies overview go further.