Crab Champions

Crab Champions is a fast paced PvE shooter with roguelike elements. You basically are a crab and fight through multiple waves of enimies. You collect loot to become stronger, and there are fun boss fights. It supports co-op multiplayer, and is made by a single indie developer.

store.steampowered.com/app/…/Crab_Champions/

Linux compatibility is great: www.protondb.com/app/774801