Arm Cannon

Poky-Armando was already running towards the mass. "Spark notes version: it's a big angry mass of fat with like, a hundred brains inside it. Don't breathe the yellow cloud, and do not let the sludge it throws touch you or you're a goner. Me, stick your arm in the back end of the gun and it'll clamp to your shoulder. Then just pull the trigger. I've got a plan to beat this thing, just need time to set it up. That's where you two come in."

Ruslav nodded, while Young Armando loaded the gun.

...

Young Armando cocked the gun, the barrel squeezing tightly around his right arm. Literal hot lead in the shape of a limb flew out at twice the speed of sound, and a new arm grew and loaded itself into the firing chamber. Rinse, repeat. The blob swung a pillar of fat at him, and Ruslav melted it with a burst of the Igneous Aether.