Nice bit of fun. 455 FtW! ...I was kinda getting bored at that point, lol. Maybe make the zombies get faster over time, as well as having more hit points.
The damage feedback could stand to be better, particularly on a per-hit basis. Maybe even just a little particle every hit. As is, it felt like I was just doing occasional damage in an aura instead of continually slicing. And I could hardly tell when I got hit myself, that should have a special sound and particle effect.
The movement ended up looking and feeling very floaty. Maybe increase the force and the drag.