Howdy! This guide tells you how to find, challenge, and hopefully defeat Chapter 4's secret boss. I'll also be giving advice on how I got through most of the boss's attacks.

Before anything, this contains spoilers (duh). Do not read on unless you've found and defeated a rather spooky boss in the First Sanctuary. The sections past 1 also contain spoilers for elements of the fight, so don't read those either unless you've already challenged it. Or not, I'm not your boss.

Alright, now that that's over with, let's get on with it.

Chapter 4's secret boss is the spirit of Gerson Boom, the Hammer of Justice. You may recognize him from Undertale, but either way, he'll use attacks reminiscent of Undyne's from that game, turning your Soul green and forcing you to play the world's most dangerous round of DDR. When your Soul isn't green, he'll pelt you with a barrage of hammers and other things that may remind you of a Mario game.

As far as secret bosses go, the Hammer of Justice isn't the worst Deltarune has to offer. Yes, his fight is very long, and your healing is much worse for reasons we'll get into later, but all his green attacks are always the same, and restarting is very easy - it takes around five seconds at maximum, or fifteen if you're coming from the SAVE point in Gerson's study. Which, to be honest, is why I grinded it out.

First off, how do we even attempt to challenge Gerson?

It may be obvious, but in order to start, you'll need to be in the First Sanctuary. If you've left to the other Sanctuaries, sorry! As far as I know, there's no way to rechallenge the boss.

Look around the Sanctuary for Golden Piano Notes. These are fairly well hidden, but not impossible to find on your first playthrough even blind - just do some backtracking after you find the Claimb Claws from Mr You're Taking Too Long, since it'll provide you with access to practically all of the Sanctuary.

First, you'll have to find the Golden Piano in the first place. This shouldn't be too hard - It's at the tippy-top of the main tower library. Besides it, you'll find a plaque that says,

Melodies three, there be.

Two in the southern towers

One where gold strikes water.

The "two in the southern towers" will require a little backtracking - go to each bottom end of the Sanctuary and look for a place to climb up. There's you'll find some little puzzles for you to do in order to get the Golden Piano notes.

The "one where gold strikes water" is easier - just fiddle around in the donation waterfall room to the left of the entrance library north of Gerson's study. You'll find something you may enjoy and a cute cutscene behind gaming's most popular method of hiding secrets.

Here's some video guides on how to get to the areas from Gerson's study:

Getting to the Golden Piano. Providing context, and now videos, since 2023.

Getting to each southern tower.

Getting to where gold strikes water. Or doesn't, if you're stingy.

I've already filled out and explored each area, so there's no spoilers for the events there.

Neatly, if you can't be bothered to go and do all the piano notes (but you really should, they're fun) you can just grab the code from the Golden Piano video above and it'll work anyway.

Once you've got all of them, head to the Golden Piano and give it a play. A super secret door will open to an atrium containing the Magic Axe of Legend!

Man, what a great axe! Would be a shame if it cost $999, 999, 999!

Unfortunately, even the Magic Axe of Legend isn't safe from capitalism! To obtain it, you'll have to pay the shop owner, Gerson Boom, more money than I think the entirety of Hometown owns! Or just give him a service in return. How does a haircut sound?

If you're tired of farming for the Spincake, burgers, tea, juice, the Shadow Mantle and all that stuff just to have a chance at beating the other secret bosses, you'll be pleased to know that you won't need items for this fight! Just a weapon (sort of) and armour.

The best gear possible for this fight is the Devilsknife, Dealmaker and Mystic Band / Shadow Mantle. This is because the Devilsknife reduces the TP cost of Rude Buster and increases your magic substantially, the Dealmaker also significantly boosts defense and magic, the Mystic Band provides a shred more healing when using OKHeal, and the Shadow Mantle heavily reduces the damage you take from a certain attack as well as apparently providing decent all-round defense.

For people who haven't gone through the lengthy procedure of banging your head against a wall for five hours straight (and if you've beat the other three secret bosses Gerson should be much easier due to your mechanical skill anyway), the best gear available in the chapter is debatably the Absorb Ax, Mystic Band, and WaferGuard.

Here's how to get to all of them:


This axe heals you for 10% of your maximum health on successful attacks. That's... not going to be relevant in this fight, but even with the boss's unique hit mechanics, you'll still heal two health on swing. Doesn't sound like a lot, and it isn't, but the boost can be very useful for scraping your way to the next turn, and all the other alternatives are useless. Yippee!

To find it, head to a library with two pianos north of Gerson's study. You'll need to maneuver both the blue and green pianos to create a bridge leading to a chest on top of a bookshelf, in which is the Absorb Ax.

Coming straight from Gerson's study!


This boosts your magic by 4, which is decently helpful for healing during the fight. It gives +8 health per heal spell, which, again, doesn't sound very significant, but chip healing is very important in this fight.

It's found in an antechamber off to the left of the room you found the AbsorbAxe in. You'll only need the green piano for this one.

You can't get this from an egg!


This boosts your defense by 4. It's found in Gerson's study on his table, which means in order to get it you'll need to pay him 900 Dark Dollars. Hey, maybe he'll go easier on you if you bribe him with enough money.

The show that's frankly three seasons in too deep and should probably have its budget pulled!

Some more notes on boss preparation:

As said before, items are useless! Don't make the same mistake I did!

Sick fruit juice. It would be a shame if I couldn't drink it.

Hey, at least they were great for the final boss of the chapter. Good luck with that, by the way.

Finally, some people have reported that controllers work much better for this fight than keyboards, so if you have one (don't spend 30 dollars on a controller just to beat Franklin the Turtle), it can't hurt to give it a try.

Wiki page on the fight,[deltarune.wiki] as the source for most of the content on this page. I appreciate you, dataminers <3

Gerson's fight is won after you survive his Special Attack™. To get to said attack, you need to get 84 stars. Stars are granted through the following actions:


From this, you can see that even if you just guard the entire fight, Turns 17+ will give you enough stars to progress anyway. However, even though the objective of the fight is to survive, using Guard every turn isn't a very good strategy - the more turns you churn through, the more chances you have to get hit and lose, so you should try to get through the fight fairly quickly. Of course, guarding isn't useless - it still provides defensive benefit, and guarding before an attack you know you'll have trouble with is helpful.

While Rude Buster only gives 6 / 8 stars and OKHeal gives you 9 stars, Rude Buster takes 50% TP while OKHeal takes at minimum a staggering 80% TP. I suggest getting off one or two Rude Busters at earlier turns that you're more confident in before swapping to using OKHeal whenever possible.

Now, let's take a look at the fight:

Gerson has some interesting interactions with some of your tried-and-true fight mechanics. As you may have seen from the intro cutscene, this wily old turtle isn't slow as a... tortoise, and will dodge any attack that Susie throws at him.

Gyah ha ha!

And as for Susie's famous Rude Buster, he's got an answer to that too:

All it takes is a right click at the right time.

Yep, Gerson can parry your Rude Buster shot, potentially dealing damage to you instead. Of course, Susie isn't going to take that lightly, and she can parry the Rude Buster shot right back at him in a fun and totally not lethal pre-turn minigame. With every shot, the amount of parries you'll have to do increases (up to a cap), so be prepared.

Anyway, this is a test of endurance, right? So all I have to do is to use items after every attack to recover and I'll be goo-

Hey! Even the skeleton didn't do that!

When you use an item, Gerson will pop up and nick it (don't worry, he gives it back later). This means no healing items or TensionGems for you - for healing, you'll have to rely on Susie's Ultimate Healing! Or as the game calls it, the OKHeal.

In the Hammer of Justice fight, OKHeal will heal Magic × 2 + 52, and its TP cost will decrease with each fight down to a minimum of 80%. Not very versatile, but it'll do.*

But enough on that, let's talk damage, which you'll probably be eating a lot of in the fight!

Gerson will deal damage (wow) proportional to your current health, so if you're at high health he'll deal more damage, while dealing less damage when you're at lower health. I believe the minimum damage that you can take is 5 when you're around single-digit health.**

As such, you'll probably stagnate around 50 HP to 100 HP for the majority of the fight once the first few hits start rolling in. Don't be discouraged - I've survived attacks on scraps of health before, and you can be safe entering even the Hammer Spam attack on 30+ health if you're confident in your dodging abilities.

This also means that you can intentionally go into a difficult attack on low health so you take less damage from things you know you'll get hit by, and heal up afterwards so all your potentially healed health isn't washed away instantly. And vice versa - even if it is made worse by the fight's damage scaling, more health means more hits you can take.

*personal complaint, but I wish you were allowed one free item before Gerson steals the rest. yes, skill issue, yes, the inability to use items represents Susie's growth and improvement in her healing skill, but it would help make keeping your health level in later turns when your TP is low more consistent while still not trivializing the fight, and what lesson isn't made better by a little break for a drink or a snack to soothe the mind and body?

**The maximum damage taken is not 48 - the first attack you eat will always deal above 50 damage. Maybe the wiki is talking about base damage?

Okay, now for what you're probably actually here for: advice on dodging attacks. I'm not going to cover every single possible one, but I'll cover all of the mandatory ones and Turn 15.

These attacks are organized based on rematches. Your first few attempts at the fight will be different, and my Turn 1 is what the wiki page calls Turn 3 (so if you want to see the wiki's advice[deltarune.wiki], add two to my turn number and look under List of Attacks).

Speaking of the wiki, it has the exact list of inputs for the green attacks there as well. It's a headache to look at and I don't want to rip it off seeing as I've already ripped off enough, but if you want to take a gander it's there.



Introduction.

Retest.

Eight sides.

You may be wondering why I slammed three attacks into one section. Well, it's because after hours of practice, beating them will be second nature to you, and there's not a whole lot to talk about when it comes to them.

Instead, let's talk about some general mechanics:

Use your movement keys to shift Susie's axe around to block arrows. The closest arrow to you will turn red, so keep an eye out for it.

For arrows that don't come from the four cardinal directions, hold both movement keys to block them, so for example the attack shown above requires you to hold Up and Left to block it.

You may notice that if you time your block carefully such that your axe moves just before an arrow hits it, it'll play a different sound, create yellow sparkles, and give 1% TP instead of 0.5%. This is a "perfect block", which is very satisfying to pull off, but make sure not to become too overconfident - it's very easy to get hit because you moved too early or too late in later attacks.

Note that you can also repress your movement key to activate a perfect block.



Hammer.

This attack will start off with a menacing hammer threatening you from the top. Ignore it - it'll hit you last. Instead, shift your axe to the bottom to get ready to intercept the arrows that come from there.

When the hammer finally hits you/your axe if you blocked it, it'll break your axe, turning you red and spawning a wild Gerson from the top that you'll have to dodge by quickly moving to the left or right.



Star Shell.

In this attack, Gerson will launch a big copyright-free shell at you that trails stars behind it. After he's had enough, he'll slam it down, causing a rain of stars all over the battle box.

Hurray! Your soul's red again! Travelling in a rough triangle pattern in this attack seems to be the way to go - stick on the end of the star trail, but always keep an eye out for Gerson in case he suddenly shifts its trajectory.

This attack is the one that the Shadow Mantle'll help you a lot more in! It provides a resistance against Dark/Star attacks, which this attack features: you'll take less damage from the star trail and star rain.

For the tail end of the attack, when the stars rain down on you, try to weave between them. It's hard, but not impossible, and it'll be good practice for the next attack:



Hammer Spam.

I think the name speaks for itself. Gerson flips the battle box and spews a flurry of hammers reminiscent of another turtle enemy in video games.

I'm half convinced that there's no consistent way to dodge this, but I found that for the smaller hammers, weaving in between the larger gaps helps, while for the larger ones staying off to the side can be helpful. And of course, for the big smashing hammer, fly off to either the left or right corner. Above all, try to keep your position relatively level - don't go flying all over the battle box, and keep an eye on the area above your soul so you can react more quickly.

It's a bit counterproductive, but since Gerson's damage scales based on your health, it can actually be more helpful to go in on low health and pop a heal afterwards as compared to healing beforehand.



Sneak Attack.

This turn is what I like to call the "Sneak Attack", because of the initial arrow barrage coming from the top leading you into a false sense of security that the small barrage of arrows coming from the bottom can take advantage off.

And like most sneak attacks, it's only really effective the first few times. Subsequent fights in which you know what you're doing can make this attack a helpful breather and a good practice for your perfect blocks (but don't get cocky).



Green Shells.

This turn introduces green shells, which bounce off your axe on the first block and have to be reblocked a second time. Before that, there'll be a barrage of arrows from multiple directions just to keep you on your toes.

Remember how I said that memorizing the exact list of inputs is a headache? Well, it still is, but doing it just for the shells isn't a terrible call to help out your muscle memory, especially since they seem to do more damage than the arrows.

Here's the list of inputs for the shells specifically:

Up, Up, Left, Right, Left, Right.



Green Shell Spiral.

This attack starts off with a few green shells before treating you to two more, with a healthy serving of arrows spiraling it that you'll have to block while the shell bounces back from your axe. For dessert, more arrows.

Here's the list of inputs for the shells at the start:

Up, Left, Right, Down, Up, Left, Right, Down.

This attack is a tad overwhelming, but feels cool when you pull it off. Don't let your guard down when the shells have been dealt with - the arrows can catch you off guard easily.



Gerson Jumpscare.

This is another "hybrid turn", in which you'll be both green and red. (Turn 4 is also a hybrid turn, albeit a very short one.) It starts off with three shells (Up, Left, Down, Up, Left, Down) before smashing you with a hammer from the right and breaking your axe again, turning you red and giving Gerson free reign to harass you from the top of the battle box.

At the end, there'll be a fakeout with two Gersons threatening to cover the whole battle box, only for a third one to come in with a steel chair horizontal strike. Don't be fooled - stay in the middle, and try not to trap yourself at the top or bottom of the battle box by accident.

This is a more traditional Undertale/Deltarune attack, just much faster. Slide around the battle box, and save for the final attack, hanging out around the bottom can give you more time to react to Gerson appearing.

Whack-A-Soul.

In this turn, Gerson will stretch out the battle box and show you what he thinks of keyboard mashing. When it starts, move to the right, then move to the left, then react to the hits as they come.

This turn is a good breather - the calm before the storm, in a way. This does NOT make it a pushover - getting hit is still a very present danger, but it's easier to master than the other turns and reattempts can consistently shrink your hit count down to below two.

However, the price of this easy turn is a lower TP gain, which you should account for.



Blue Shells.

This turn introduces blue shells, which will move two spaces to the left when they're blocked (so the shell that I'm about to block will move down to South, while the shell on top will move to North-West if I block it). Immediately afterwards, you'll be accosted by several more blue shells and a flurry of arrows. Wonder where we've seen that before.

I'm not gonna sugarcoat it, blue shells suuuck (thanks, Mario Kart). Do your best and try to learn the timings for them, but if you must get hit, exploit your immunity frames to get hit by other projectiles that you may have trouble blocking. Chin up- if you've been consistently fighting and healing, the end should be coming soon.



Blue Shell Spam.

Some green shells and arrows (Left, arrows, Right, Left, arrows, Right) before more blue shells, just for you! You'll have to juggle three of them at once, twice, before a final rain of arrows from the top and a hammer to finish it off.

I'd suggest trying to get used to the trajectories that the blue shells will take and moving your axe there preemptively. Tough, seeing that this is practically halfway/almost all the way through the fight depending on what you're doing, which makes practicing this attack take significantly longer than most others, but do your best. Watching no-hit guides was very helpful for me, and it may benefit you too.



Arrow Spiral.

I hope you're good at turning your axe, because this attack launches a few spirals of arrows at you that you need to rapidly swivel your axe around to block.

Try not to panic. This is the biggest test in whether you're good at manveuring your axe, because just pressing your movement key in reaction to the arrows coming at you isn't going to cut it. Interestingly, I found the latter part of the attack where you have to essentially mash your movement keys to be easier than the slower start.

The good news is that passing this attack grants you 30 whole stars, which, if you've been fighting and healing, easily pushes you into:



Last Turn. Good luck!

Congratulations! You're at the final attack! It'll start off with a series of multiple shell colours, including a red one, which takes four blocks to fully destroy. For the grand finale, Gerson will treat you to another round of hammer slams, this time from all directions of the battle box, not just the top.

The advice for this attack is a combination of all the other shell advice- predict where they're going, and if you need to, exploit immunity frames. At the end, travelling in a counterclockwise direction allows you to evade Gerson's Hammer Slam Supreme. Just don't make the same mistake I did in the first clip. Please.

Well, that's totally all of the turns that this fight has to offer! I'm sorry, I'm not doing Turns 16, 17, and 18 because my sanity's already been torn to shreds :(

Here's my full fight footage:


This attempt had better plays in the first few turns, but, well, I think the ending speaks for itself.


I had much worse attempts at the first few turns in this one. And of course it's the attempt I succeed in...

(fight ends at 4:40. fitting)

And because I know you little freaks like them, here's my personal tier list on the attacks:

Ordered within tiers from easier (left) to harder (right). Insert mandatory disclaimer that this is just my opinion here.

Well, we're just about done here! You can go if you want, but have some complimentary tables recording all of my losses if you're interested.

(I promise you I didn't mean for exactly my 101th attempt to succeed. If anything, I would've preferred the 100th attempt ;-;)

Here's the main data sheet:

And a turn loss bar chart sorted by increasing turn number.

The same bar chart, just sorted by increasing loss number.

Some things to note:

There are a few "plateau" turns that I consistently lost on, namely Turn 6, Turn 9, and Turn 14. Keep an eye out for those.

I lost on Turn 6 the most, although the losses on Turn 15 & 19 hurt even more knowing that I was so close to the end.

All in all, this whole ordeal took a week's worth of attempts and five in-game hours to beat.

So why am I telling you all this? It's to impress upon you that if I can do it, with all my terrible losses and skill issues, with a pinch of determination, so can you! Maybe. Either way, good luck, and shave Gerson good for me!

Syringe