The anihilarg from BEN 10 Omniverse,I think that’s what it’s called. The device is able to destroy a universe,while Viltrumites struggle to survive on the surface of the sun.Since the sun isn’t destroying the universe,the anihilarg could kill them all.
The omnitrix
The mask from The Mask. Toon force is too op
The Death Star Laser. Mark said in the alternative universe, that Omni-man was killed by experimental quantum bombs, and that it costed them most of europe. This assumes these bombs can kill a Viltrumite, but not destroy a planet, so by this logic, any planet-destroying weapon should do the trick.
If that one mortal kombat line is to be believed and omniman really can operate at absoloute zero you can just forget about energy weapons because he can survive in an environment in which molecules are not moving at all. No motion whatsover.
Necron gauss weapons
Orange doom rocket from The Dictator movie