Slay the Spire 2 Final Boss Guide: How to Beat Test Subject

Why This Boss Ends So Many Runs

Test Subject is the hardest fight in Slay the Spire 2. It has 600 total HP across three phases, and each phase punishes a different weakness in your deck. I’ve watched strong runs crumble here because the deck was built to do one thing well — but this boss demands that you do three things well.

Most Act 3 Bosses test one part of your game. Doormaker disrupts your hand. Queen tests raw output. Test Subject shifts the rules mid-fight. It punishes you for playing Skills in phase one, punishes unblocked damage in phase two, then cuts your damage to 1 every other turn in phase three.

Here’s how to handle each phase and what your deck needs to actually survive the full 600 HP gauntlet.

Phase 1: The Enrage Sprint (100 HP)

Phase one looks deceptively simple. Only 100 HP. You should be able to tear through that, right? The catch is Enrage 2. Every time you play a Skill card, Test Subject gains 2 Strength. Two skills? That’s +4 Strength on every hit it throws at you for the rest of the phase. Three or four skills and those counterattacks become lethal fast.

What it does: Alternates between Bite (direct damage) and Skull Bash (damage plus debuff). The damage numbers start low, but they scale rapidly if you feed it Strength through Skills.

How to play it: Attacks only. Seriously. Pretend your Block cards don’t exist for 2-3 turns and just race the boss down. 100 HP goes fast when you’re swinging with your full hand of Strikes and attack cards. If you absolutely must play a Skill — maybe you’re at dangerously low HP — keep it to one per turn maximum.

This phase is a DPS check. If your deck can’t deal 100 damage in 3-4 turns using mostly attacks, that’s a sign your offensive scaling was too slow coming into Act 3.

Phase 2: The Block Wall (200 HP)

Kill the first form and it immediately revives with 200 HP and a new modifier: Painful Stabs. Every point of unblocked damage shuffles a Wound into your discard pile. Not one Wound per hit — one Wound per unblocked damage instance. Take a 15-damage Multi-Claw without blocking and you’re suddenly drowning in dead draws.

What it does: Uses Multi-Claw every turn. Starts at 3 hits, and gains +1 hit each use. By turn 4, it’s hitting you 6+ times per turn. Each hit is moderate damage, but they add up. And every unblocked hit dumps more Wounds into your deck.

How to play it: Block everything. I mean everything. Even 1 point of unblocked damage creates a Wound. Your draw quality degrades fast once Wounds pile up, and a clogged hand means less Block, which means more Wounds. It’s a death spiral if you let it start.

This phase flips the script from phase one. Now you need those Skill cards — Block, Defend, defensive powers. The Enrage modifier is gone, so play freely. Front-load your defensive setup here. If you have Powers like Barricade, Frost orbs, or After Image, play them immediately.

The multi-hit nature of Multi-Claw means that flat Block per hit (like Flame Barrier or Thorns effects) gets extra value. Each hit triggers the retaliation separately.

Phase 3: The Intangible Marathon (300 HP)

This is where most runs actually end. Phase three drops Painful Stabs but gains Nemesis, which grants Intangible every other turn starting with the first turn. When Intangible is active, every source of damage — no matter how large — only deals 1 damage.

Your 50-damage heavy attack? Deals 1. Your 200-damage poison tick? Deals 1. Intangible doesn’t care about your numbers.

How to play it: Your turns split into two categories. Intangible turns: block and set up. Play Powers, apply debuffs, stack Strength. Vulnerable turns: dump everything into damage. The pattern is predictable — turn 1 Intangible, turn 2 vulnerable, turn 3 Intangible, and so on. Plan your energy around this cycle.

Multi-hit attacks gain value here. Each individual hit deals 1 through Intangible, so a Shiv volley hitting 8 times deals 8 damage instead of just 1. Not huge, but chip damage on Intangible turns adds up over a long fight.

300 HP at half-speed is an endurance test. If your deck can’t sustain offense and defense for 10+ turns, you’ll run out of steam before the boss runs out of HP.

Deck Preparation: What to Bring

The biggest trap with Test Subject is building a one-dimensional deck. A glass cannon murders phase one and dies in phase two. A pure tank survives phase two and times out in phase three. You need balance.

Scaling damage is mandatory. Demon Form, Noxious Fumes, high Focus with Lightning orbs, Star generators, Doom stacking — anything that grows stronger over time. Flat damage cards that hit for 12 on turn 1 and 12 on turn 10 won’t cut it against 600 total HP.

Passive defense wins this fight. Frost orbs, After Image, Plated Armor, anything that generates Block without you needing to draw and play specific cards. Phase three’s Intangible turns give you breathing room to play Powers, which means passive defense has time to come online.

Be careful with debuff-heavy strategies. Phase transitions can reset your progress. Your carefully built Poison stacks may not carry between forms. Poison and Doom can still work, but stack aggressively within each phase rather than relying on slow accumulation across the full fight.

Save your potions. A Strength potion on a phase three vulnerable turn can shave off 30-40 HP in one burst. A Block potion during early phase two Multi-Claws prevents a Wound cascade. Don’t waste them on Act 3 hallway fights.

Character-Specific Tips

Ironclad: Demon Form early, suffer the setup cost, and by phase three your Strikes hit hard on vulnerable turns. Barricade lets you stockpile Block during Intangible turns. Flame Barrier punishes phase two’s Multi-Claw because each hit triggers retaliation separately.

Silent: Wraith Form buys free setup time in any phase. Sly discards keep your hand clean when Wounds pile up in phase two. Noxious Fumes (a Power) ticks every turn and survives phase transitions — pure Poison stacks don’t. Shiv builds chip through Intangible with multi-hit damage.

Defect: The best character for this fight. Frost orbs handle phase two defense passively. Lightning orbs chip through Intangible. Once the orb engine runs, both jobs happen automatically. Prioritize Defragment, Capacitor, and Glacier.

Regent: Bank Stars during Intangible turns, unload on vulnerable turns. Sovereign Blade scales into the endurance grind. Get your Star economy established before phase three or you’ll fall behind fast.

Necrobinder: Phase transitions cleanse Doom, so treat each phase as a fresh Doom race. Osty attacks passively regardless of hand quality. Bury effects live in your deck, not on the boss, so they survive transitions.

Common Mistakes

Playing Skills in phase one. The number one run-killer. You see a big attack incoming, panic, and play Defend. Now the boss has +2 Strength and that next attack hits even harder. Tank the hit and keep swinging. Your HP pool should be large enough from Acts 1-2 to absorb a few unblocked attacks.

Letting Wounds spiral in phase two. One unblocked hit leads to Wounds. Wounds clog your hand. Clogged hands mean less Block. Less Block means more unblocked hits. More unblocked hits mean more Wounds. This is the death spiral. Block first, attack second.

Wasting big damage on Intangible turns. I’ve done this. You draw your best attack card, get excited, play it — and it deals 1 damage. Track the Intangible cycle. Odd turns (1, 3, 5…) are Intangible. Even turns (2, 4, 6…) are your windows.

Overcommitting to one strategy. A hyper-aggressive deck without Block. A pure tank without scaling. A debuff deck that loses everything to phase cleanse. Test Subject punishes specialists. You need a deck that attacks hard, blocks cleanly, and sustains over a long fight.

Ignoring potions. Save at least one offensive and one defensive potion for this fight. They’re worth more here than anywhere else in the run.

Putting It Together

Test Subject is the final exam. Phase one tests your attack output. Phase two tests airtight defense. Phase three tests sustained endurance. Build with all three in mind from Act 1, and this boss becomes tough but winnable.

Defect and Ironclad struggle the least thanks to natural scaling. Silent and Necrobinder need more intentional construction. Regent adapts well but gets punished if Stars aren’t flowing by phase three.

For deeper card priorities, check our build guides: Ironclad, Silent, Defect, Regent, and Necrobinder.