Cool game. I lol'd at the capsule player shadow.
I like your style, the color palette, the sound design, the music, the enemies. This is good stuff. The burst-firing gun seems like it'd make more sense for a mech whereas it seems you're playing a regular dude? It is fun lining up the shots to make sure you hit a row of the enemies all at once.
Really needs a sound cue and/or something on the crosshair to show when the gun is ready or loading, it's very awkward having to look down in the corner. Maybe a click or something when it refuses to fire too. Perhaps another pickup could be an instant reload?
I really don't like the overuse of Painkiller-style arena lock-ins. But at least the arenas here are a little more open and it's a good touch to make you walk through it before you push the button to spawn everything. The railguns hits from the really big mechs are annoying, you just have to pop out to hit them, I don't know if they had a tell or anything. The other enemies by comparison are pretty easy to deal with.
I want to add some mech enemies to my game as well, this has given me a few ideas.