Ah, shit, I just realized that I could use the other elevator to go upstairs as well. Now I managed to beat the game and have some more stuff to share:

• I'd suggest letting the player exit the elevator by pressing "E". Cause otherwise if you changed your mind about using the elevator, you'll have to go to another floor and then come back, which is rather annoying. Same thing with moving more than one floor: I'd rather press down or up and continue moving instead of having to exit and enter the elevator again.
• Bug: the creature shooting at you on the first floor sometimes disappears, but still keeps shooting, so the bullets just appear out of nowhere.
• Another bug: sometimes when the boss phases out of the middle of the screen to shoot at you, his old hitbox still remains there and can damage you. In fact, after several attempts it kept staying there every single attempt.

But I don't think it matters that much anyway since the game is so short.