Fun game! I played to the finish and was motivated to keep trying when I died a few times. The controls worked well -- often times in FPSs with mouse control the screen movement can be wild, but you did a good job balancing its responsiveness. The gun also worked well -- it shot accurately from a distance and I never had the feeling like I was cheated out of a hit on an enemy, nor that their collider was too big. The overheating mechanism worked well. The colors were nice, I could track enemies well, and I really liked the 2D-ish animations when defeating enemies. You also did a good job varying the geography of the rooms so that I could remember where I was when going backwards and forwards through rooms, and you did a nice job using the sound to alert the player of enemies.
I found it refreshing to play a FPS where proceeding cautiously was good judgment. However, I did find that if I edged very slowly into the rooms with bosses, I could hit them from a distance without any resistance. This was especially true of the final boss. They might need to "awaken" when they are within range of the player gun. And it's a very minor issue, but I stumbled on the invisible colliders that tended to be in long corridors on the right and left.
Great work!