“Look out, everybody, things are about to get awesome!”
Before the Vault Hunters can fight One Punch a puzzle must be solved to get him to spawn. To do this, five screens that can be seen on the wall ahead of the puzzle must be switched on. A method that works best is to hit the floor handle twice, then spin the valve on the wall next to it.
As for the fight with One Punch, he can be quite a difficult foe to deal with due to his ability to one-hit kill the Vault Hunters regardless of their level. The best way of defeating him is to burst him down as quickly as possible. Though his head is a critical hit spot, it is hard to actually hit his head because of his movement pattern. One Punch never wears a shield so the most effective weapons are incendiary weapons. A flamethrower like the Long Musket can be one of the best choices because it wears him down quickly and it is easy to aim at him as flamethrowers deal indirect damage.
High damage weapons with bullet ricochets such as the Conference Call, Hyperfocus XZ41, Juliet's Dazzle, or Wagon Wheel are very useful in the tight hallway he enters since most ricocheted bullets will hit him again. Weapons and grenades which focus multiple splash explosions in a small area such as the Devastator, Roisen's Thorns, and recurring or cloning maddening grenades could be very useful as well as all explosions are likely to hit.
Amara is able to cause enough damage with a shotgun headshot or Tediore SMG / pistol reload to stagger him for very short periods, which helps delay his getting to the other end of the hall. He has to be fought from inside the switch room for the purposes of keeping Tediore MIRV spam from getting away from him. With an incendiary shotgun, it is likely he would be unable to reach the room fast enough to hit her if she has invested points in either Fist of the Elements' elemental damage or Mystical Assault's gun damage skills.
Fl4k may be able to use their pet as a temporary distraction while they land high damage critical shots. Since they specialize in focused damage they should have few problems with this fight.
With Zane, using the clone at the entrance to the hallway One Punch emerges from seems to be very effective; On mayhem mode 2, One Punch is unable to kill the clone in a single hit, but attacks it first, so it provides a useful distraction. Zane's Infiltrator class mod which provides a 20% weapon damage and speed increase could be useful here. Using the Double Barrel skill to give the clone a copy of the Conference Call or similar allows damage output from reflected bullets to rapidly drain his health before he is able to hit the clone a second time.
- His mechanic to one hit the Vault Hunters can be bypassed by using an artifact that renders the wearer short time invulnerable when falling to 50% hp. However this is not recommendable since the Vault Hunters will still be a two-hit kill.
- One Punch has a chance to drop the legendary One Pump Chump shotgun.
- One Punch is a reference to Saitama, the protagonist of the anime One-Punch Man.
- Unlike other unique enemies, One Punch does not level up with the Vault Hunter's level.