How to Beat Test Subject in Slay The Spire 2

5 days ago 6

Published Mar 8, 2026, 2:51 PM EDT

Laurence is an avid writer, gamer, and traveller with several years of journalistic writing experience under his belt. Having helped create a student-focused magazine at university, he is keen to reach the gaming community with his guides and game reviews.

Whether it's trudging through the world of Elden Ring, or grinding out raids in Old School Runescape, Laurence is always up for a challenge. However, in real life, his current challenge is learning how to speak Spanish and cooking authentic Mexican cuisine.

Slay The Spire 2 is here, and with it comes a whole host of ridiculously challenging bosses ready to pummel you into the dirt over and over again. Unless you’ve got a stellar deck… then you’ll be doing the pummeling!

Naturally, the final 3 bosses encountered at the top of the Spire, at the end of Act 3, are some of the most challenging enemies you’re going to face. The Test Subject is one of these 3 bosses, and is going to seriously test your defences, as well as your ability to dish out damage extremely fast!

Let’s take a look at the unique mechanics of this fight, as well as some of the best methods to send this beast back to the lab!

Encounter Mechanics

One of the first things you’ll notice about the Test Subject (apart from its randomised name) is that, once you’ve killed it a single time, it will sprout a new head with 100 extra hitpoints. This happens twice over the course of the fight, meaning at Ascension 0, you need to deal a total of 600 health.

That doesn’t seem so bad, right? Well, unfortunately, that's not everything this multi-headed monstrosity has to offer. Each phase has its own unique buff:

In phase 1, the Test Subject will have Enrage, giving it +2 Strength every time you cast a skill.
Phase 2 has Painful Stabs, which inflicts a wound every time the Test Subject deals unblocked damage.
Phase 3 has Intangible every other turn. Monsters with intangible will only take 1 damage at a time. You might remember this mechanic from the Nemesis in Slay the Spire 1.

The first 2 phases are pretty easy to deal with, but it’s the 3rd that really ramps the difficulty up. This intangible mechanic will make this phase twice as long, seeing as how you will barely make a scratch every other turn. This means you need high burst damage when it’s tangible, and good defences to keep yourself alive when it’s intangible.

Boss Strategies

Before we go into what actually works, there are a few builds that might struggle with this boss. Certain decks that rely on building up a ton of debuffs, like Poison and Doom, are going to struggle with this boss. They are generally better against enemies with a single large health pool.

When the Test Subject enters a new phase, you’ll lose all of those debuffs you’ve placed on it, basically meaning you’ll need to start from scratch.

Instead, you’re better off going with a heavily defensive build, as this fight can last for quite a while. Frost orbs, Barricade, Afterimage, Reanimate, and other defensive options can help a lot here.

Look out for powerful relics offered by the Ancients. Certain relics like Jewelled Mask, Storybook and Jewellery Box can offer your deck a serious power boost!

If you’re not much of a defensive player, heavy-hitting builds can be pretty great. Anything that scales over time is nice, too! Demon Form Ironclads, Sovereign Blade Regents, and Bury focused Necrobinders won’t have much trouble here.

When you pair these builds with Vulnerable and some decent energy generation, you’ll have each of these phases down before they can pose a threat. Burst damage is the name of the game here.