The last attack in the combo of the Sponge Striker sometimes hits for more damage than the game can count. The damage number overflows the same way 'total damage' does sometimes at the end of a level, displaying as -1,000,000,000+.

This isn't just a visual bug, when this happens it wipes the Mooshroom Monstrosity's entire health bar in 1 hit.

I'm not too sure how the unique perk on the weapon works, but i'd wager it's something to do with that.