Well done on making a multiplayer game! I think there is an interesting idea here, where maybe if I could keep my opponent busy they will never be able to rest and maybe I'll be the only one who can upgrade. I was only just playing against myself in two different tabs though so a bit hard to test. I did get up to the maximum upgrade level and then it was fun to machine gun my enemy down all at once. Because the camera is quite zoomed in I would go right up to them and shoot from point blank range so I could see the bullets land.
Good job figuring out how to code all of this in 7 days!