Hello!
I was playing your game and I must say, it's quite good. I like the style of the game and would love to see the game grow. Although I think some changes could be made in the future.
First, a lot of people agree that the game is way to hard and I think so too. I don't think a change should be made for Medium and Hard but I think making Easy modes is needed. Another that should be changes is the amount of zombies that come from the tombstones. A couple times 150 zombies would come out of one and well, they took a lot of time. Another thing I find hard are the Tank zombies. I don't know if they were intended to be really hard but I seem to die ALOT from the hundreds of zombies they spawn. Finally, I was playing for a while and had to fight 550 zombies, surprisingly, I won the battle but for some reason it was staying on the same stage (shown in the picture above) . It was probably because There was WAY to many zombies and the game probably couldn't handle it.
Anyways, hope this helps!