Learn how to beat the Hammer of Justice in Deltarune Chapter 4. Discover the best equipment, why you should ignore Rude Buster, and how to survive every attack.
If you are pulling your hair out trying to figure out how to beat Hammer of Justice, you are not alone. Deltarune Chapter 4’s secret boss completely upends everything you know about combat. You are forced into a solo battle as Susie against Gerson’s Darkner incarnation, combining brutal Undertale-style Green SOUL mechanics with highly deceptive damage scaling.
By tweaking your gear, understanding the hidden progression mechanics, and ignoring some of the game’s most misleading hints, you can turn this bullet hell into a manageable memory game. Here is the complete breakdown of how to prepare for and dominate the Hammer of Justice.
Preparing for the Deltarune Gerson Boss Fight
Before you even worry about dodging arrows, you have to unlock the fight. Gerson hides the JusticeAxe behind a massive paywall of $999,999,999 before offering a “haircut” challenge that triggers the battle.
How to Unlock the Secret Boss (Golden Piano Puzzle)
To access the hidden fireplace door in the Dark Sanctuary Study, you must collect the ClaimbClaws by defeating the Jackenstein mini-boss. This climbing gear lets you scale book-ladders to find three hidden music sheets:
- Music Sheet 1: Head to the lower western path past the Study. Climb the wall near the patrolling puffballs and defeat the HolywaterCooler (Miss Mizzle) mini-boss.
- Music Sheet 2: Walk down the stained glass hallway and scale the climbable wall. Navigate the pitch-black area to find a glowing figure, follow their path, and hit the button to reveal the sheet.
- Music Sheet 3: Head west through the library and go behind the waterfall. Navigate the dark maze to a climbable wall of bells; the music is at the end of the next section.
Play the collected notes on the Golden Piano in the upper Study to open the fireplace.
Best Equipment for Hammer of Justice
Standard defense stats are practically useless here because Gerson deals percentage-based damage. Instead, you must maximize your magic stat to supercharge Susie’s healing abilities.
- MysticBand: Found by manipulating the blue and green pianos to create a bridge in the piano room. This boosts your magic by an essential +4.
- Dealmaker or Jevilstail: If you have the Dealmaker from your Chapter 2 pacifist run, equip it immediately. Otherwise, use the Jevilstail.
- AbsorbAx: Equip this weapon (found in the chest on top of the bookshelf near the piano room) to give Susie a slight, much-needed health buffer.
With this magic setup, your OKHeal spell will scale drastically, eventually restoring up to 76 HP per cast.
The Core Hammer of Justice Deltarune Strategy
Your success relies entirely on abandoning your usual combat instincts. You cannot damage the Hammer of Justice. Instead, you must survive long enough to amass 84 hidden “stars” to impress Gerson and trigger his final attacks.
The Hidden Star System: Always Attack
It might feel wrong to swing your weapon when the boss dodges every hit, but attacking is strictly required.
- Attacking generates 24 TP, which is significantly more magic than defending.
- Every attack earns you 4 stars toward the 84-star goal.
- Defending simply wastes turns, earns zero stars, and leaves you starved for healing magic.
Master Your OKHeal Timing
Perfecting your TP management is what separates the winners from the players stuck on attempt number forty. Gerson’s damage scales down as your health drops. When you are critically low on HP, his massive hammer strikes will only deal about 5 damage per hit.
Wait to use your OKHeal spell until your health is in the red. Because the incoming damage is so low at that threshold, healing at the last possible second gives you the absolute maximum value. Better yet, every successful OKHeal grants you 9 stars, rapidly accelerating the fight.
Do You Have to Use Rude Buster? (The Trap)
The game strongly pushes you to use Susie’s signature spell by introducing a unique deflection minigame. When you cast it, Gerson deflects it back, forcing a high-speed ping-pong match.
- Successfully counter-deflecting earns 8 stars.
- Failing to counter-deflect earns only 6 stars.
While it seems necessary, this mechanic is a massive Rude Buster trap. Casting the spell drains the TP you desperately need to keep yourself alive. Because OKHeal grants 9 stars and keeps you breathing, ignore Rude Buster entirely and save your magic for survival.
How to Survive the Attack Patterns
Gerson uses the exact same attacks in the exact same order every single time.
The Green SOUL and Colored Shells
You are locked into directional blocking, but Gerson throws multiple projectile types at your shield:
- Green Shells: Must be blocked twice.
- Blue Shells: Must be blocked three times. Crucially, they always jump exactly two spaces to the left after a block.
- Purple Shells: Must be blocked four times.
- Red Shells: Must be blocked five times.
- Hammers: Instantly turn your SOUL red, forcing you to dodge freely rather than block.
How to Dodge Bouncing Shells (Turns 13 & 14)
When the screen fills up with blue shells, stick to one golden rule: move your SOUL in continuous, large circles or wide diamond patterns. Because the blue shells always jump two spaces, predicting their trajectory is easier if you constantly sweep your shield in a circular motion rather than reacting to individual hits.
Surviving the Arrow Spiral (Turn 15 and Beyond)
Late in the fight, the standard four-sided combat box expands into an octagon, bringing the arrows in from eight different directions. You have to rapidly swivel your shield to block spiraling volleys.
- Keyboard vs. Controller: If you are playing on a PC, strongly consider plugging in a controller. Using a thumbstick makes defending against the eight-directional diagonal attacks infinitely smoother than trying to mash dual keyboard arrows.
- Take the Hit: If you know you cannot block a complex arrow pattern, deliberately get hit by a weaker projectile to exploit your invincibility frames and pass straight through the heavy damage. Surviving these late-game attacks naturally grants massive star bonuses (Surviving Turn 17 alone gives 30 stars), pushing you instantly to the finish line.
The Rewards: Why You Need to Beat Him
Enduring this gauntlet is well worth the frustration. Taking down the Hammer of Justice rewards you with some of the best late-game loot available in Chapter 4:
- The JusticeAxe: Grants a massive +12 Attack for Susie.
- Permanent Magic Boost: Susie’s base Magic stat is permanently increased by +4.
- BetterHeal Upgrade: Permanently improves the healing formula of your spell and reduces its TP cost by 5%.
- The Shadow Crystal: The highly coveted collectible required for the game’s overarching secret narrative.
Keep your magic stats high, attack constantly to build your TP, and only heal when you are clinging to your last bit of health. Taking a break for a day and letting your brain process the bullet hell patterns often leads to beating it on the first try.
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