I a few suggestions that would significantly improve the playability of this game mode
- Zombies should spawn off-screen rather than right on top of you
Reason being is that it's incredibly frustrating to have something spawn directly on top of you then die because you had no idea where the zombies spawn and I can understand the concern if a group players were spread all across the map preventing zombies from spawning. That is why you have defined spawn location either outside of the map or in an easily identified areas like a manhole. I also can understand if you might think people will exploit these spawn locations which is reasonable but there are so many enemies, and they're spawning all around, it doesn't matter.
- The boss needs to be reworked
The boss is too easy; I'd rather have a boss with a lot of health and some pretty devastating attacks if you are not careful, rather then have a boss with next to no health and the only real defense it has is the absurd amount of zombies creating a meatshield to block the gunfire, just sayin'.
- The upgrades are underpowered
The upgrades system doesn't need an overhaul; it just needs a lot of tweaking. There are only a few upgrades worth purchasing that being your special ability, extra lives, and health, that is it, the rest of the upgrades contribute very little to improving your character. You don't need the damage upgrades since the zombies are going to die in the same amount of hits regardless of how much you upgrade it. You don't need to level up your speed since you already run faster than every enemy in the game. Combo duration is useless since you get your combo so frequently you pretty much have it active at all times. The ammo upgrade is probably the most useless because you get weapons frequently enough to the point where you don't need to hold onto a gun for more than ten seconds.
I hoped this helped or at least brought some insight to the general balance of the game mode. It is just what I think.