Huge Enemy - Worldbreakers is a rather impressive side-scrolling shoot 'em up that's officially released with Linux support, it also has a demo to try out.

Feature Highlight:

Eleven playable levels including two survival challenges and a speedrun

Longplay

Fourteen animated end-level bosses

Up to 25 000 ennemies displayed by level

Quantities of pyrotechnic effects for shots and explosions

High définition "Pixel Art" for a modern and very detailed design

After trying the demo on Steam I was pretty impressed by how completely insane the game is. Your weapons are crazy, the enemies number in their thousands constantly and the way it has things going in both the foreground and background makes it quite a sight. Enemies will fire at you from far away, you can dodge into the background to avoid fire and so on.

The controls on a gamepad are a little, uh—awkward. It feels very much like they simply tacked on gamepad aiming without really thinking it through, so it's much better with a mouse. Even with a mouse the sensitivity is too high with no option to change it in game, the only slider to adjust it didn't really help at all.

Still, this is only a demo the full game might have a few updates it doesn't. If they update the gamepad aiming and have proper sensitivity options for the mouse it would be a solid recommendation for shooter fans. Gripes about controls aside, the game does seem really quite good.

Find it on Steam with 25% off until the 14th of September.