I think the boss should have lower bullet speed, it feels too fast for a first level boss. Skeleton bombers should have explosion updated so that it actually shows the range of the explosion. Right-click should also get better sprites on explosion so that it doesnt fill up a lot of the screen. Also a cooldown bar for the right-click. Something small I think would be cool is the rooms lighting up once you finish it, it's pretty confusing when you already finished a room and you end up circling to a room you already finished.