Aw right, I'm gonna be honest: I am testing this from a literal third world laptop. Please ignore the speed-related feedback on the following paragraph in case it does not apply to a better PC.
Let me start with what I believe to be the most pressing issue: mouse look. I think I get the picture: the mechs are supposed to be heavy and slow; it follows their combat style has to be slow-paced too -- but the speed at which the player looks around is anxiety-inducing, especially when there are active threats in the room. Cranking up the mouse sensibility on the menu doesn't fix this, also making the aim noticeably zig-zaggy in the process, and the "Shift + Backwards" maneuver to do a 180 doesn't seem to work for me. I personally believe having the robots turning around quickly (at least the player, if no one else) doesn't compromise the vision I believe you have in mind as long as they keep their slow movement speed. Maybe the current mouse look behavior should be tied to "Fortress mode", allowing you to look freely when it's not enabled (sort of like the focus mechanic in Touhou).
I got used to the controls with time, but integrating them to the tutorial with more instances of in-game text rather than/in addition to memorizing the list of keys before starting the tutorial would certainly make it all the more intuitive, as would having some text quickly pop-up when switching weapons and turret items ("Minigun", "Railgun", ...). Having "Q" and "E" switch the weapons instead of the numbers wouldn't do any harm too.
The rest of suggestions all have to do with polish, which I think you're already working on. Shooting at the ground leaves tiny and huge bullet holes on Arcade and Tutorial, respectively. Healing while in Fortress mode stops the bar from being updated until you try healing with it disabled. Looking forward to the spinning minigun and particle effects for the firearms and explosions in future versions of the game.
Your HUD is a big plus in my eyes. Have you ever tried to integrate the three elements on the top of the screen (the "M", 殺 and high score counters) to the robot model? They're fine where they are, by the way, but you could always go all the way in emphasizing the unique points in the project. I get the impression the radar has to stay where it is (it's the most life-and-death HUD element, after all; you can't have that bobbing around), but its range could possibly be increased, the game being focused on ranged combat and all. The character portraits, of course, are cute and the story behind them is simply heartwarming! Good luck and have a great time, the two of you, going forward from here.
Playthrough: 30+ minutes in two sessions; finished the tutorial a few times (it seems something was trying to kill me in the final room, but I powered through towards the exit; later I found out it was a boss behind me; smacked 'im); goofed around a bit on Arcade mode and found the [SPOILER].