Good job for making this game. In my opinion, you can improve this game by changing/adding some game mechanics. These are the following:
Damage Delay: The enemies' sprite animation and the damage dealt to the user has a delay. It confuses the player if he got hit or not due to the delay.
Enemies' Health: It would be better to see the health of your enemies since it can help the player prepare and strategize. The buff system that you have added would be kinda pointless since it is hard to notice the buff.
Player Damage System: I think the game would become more interesting if there are some changes in the damage you dealt to the enemy based on where you shoot him. For example, shooting the head would deal more damage than shooting your legs. You can also add some features like shooting the legs of the enemy would slow them down after shooting/dealing x damage to their legs.
Enemy Damage System: I'm not sure but I think the scaling of the damage is not reasonable(?). Based on the level(knife level, rifle level, grenade launcher level), the damage is also constant. I think you can randomize the damage from each floor to have more intense gameplay. Think about it in this perspective, you got hit while fighting the last enemy of the game but you survived the damage by 1HP.
Given the time and acads, this is a pretty good game.
If you still want to improve your game and have the time to do it, you could use these advices. Overall, I enjoyed the game. It was challenging and interesting. Good Job :))).