Deltarune Chapter 4 Secret Boss Fight Guide

There’s a secret old man Gerson boss fight in Chapter 4 of Deltarune, and no, it’s not locked behind S-ranks or arcade boards like the secret boss in Chapter 3. This time, it’s you’ll want to focus on a piano, music sheets, and some good old-fashioned backtracking.

Start With the Golden Piano Puzzle

The secret boss is tied to the golden piano room you find partway through the chapter. You’ll need to collect three hidden pieces of sheet music to solve the puzzle. Check out our detailed guide on the golden piano puzzle and how to get the sheet music.

If you skip them or move too far into the story, you might lock yourself out.

Chapter 4 Secret Boss Fight Tips and Walkthrough (Hammer of Justice… AKA Old Man Gerson Boss)

This isn’t the hardest fight in the game, but it can still wreck you if you’re not paying attention. The Shadow Mantle has minimal effect, so don’t worry about that.

You don’t need to deal damage at all

This is a timed survival fight, not a test of how hard you can hit. The boss moves through phases on his own, whether you attack or not. Your best bet is to defend, build TP, and focus on dodging cleanly so you’re not wasting heals early.

He’ll turn your soul green at the start, which means you can’t dodge... but you can block. Use your axe with the directional keys or stick to block incoming arrows. The next arrow about to hit you will glow red, so watch for that and move your guard accordingly.

Don’t bother with Rude Buster.

He’ll reflect it right back, and while you can press confirm (Z or A) to deflect it again, it doesn’t really do much. Stick to defending.

Later in the fight, he starts monologuing about the “Lord of the Hammer.” After a few phases, a hammer drops into the fight after each arrow barrage. Wait until the arrows finish, then move to the left or right of the box to dodge the hammer drop.

He’ll also throw in a giant bouncing shell mid-fight. It ricochets around the box, so stick to the bottom right, center, and center right to avoid getting clipped.

Eventually, you’ll get the full axe rain.

This phase covers the top of the box in falling axes. Just pick a side and make small movements between the gaps. If you panic, you’ll probably take more hits than needed. Susie actually takes less damage when her HP is low, which buys you some time.

Susie’s heal gets stronger the more you use it

One of the biggest things to remember is that every time you use her healing ACT, it increases by 2 HP. If you stretch the fight out, it scales into the 50+ range. It’s perfect for a long survival fight like this. Play it slow, keep her healing, and let the boss burn through his own phases.

Chapter 4 boss fight tips to remember

To reiterate, Susie’s heal scaling and the Vampire Axe help a ton, especially in the later phases with hammer drops and axe storms. And yes... the boss drops a Shadow Crystal just like Jevil and Spamton.

Just survive. That’s the real win condition.

Note: Some players report seeing a “shadow version” of the fight depending on earlier choices like the tea scene with the old man, but the core boss and reward stay the same. I personally have not verified this information.

Bonus: Secret Boss Chapter 4 Video Guide

We don’t make video content, but if you want to watch some solid gameplay of the boss fight and collecting the music sheets, we recommend this video by koshkamatew 2 on YouTube.

Final Blurb

The Chapter 4 secret boss is tied to a music puzzle. If you missed a sheet by going too far into the chapter, you may need to start a fresh save… But it’s worth it.

This fight hits harder than it has any right to... and not just in damage numbers.

If you're doing a secrets run, don’t skip this one.