Using the l33t exploit mentioned by Jobiface, I found this game to be a breeze. Nothing like an auto-shotgun that can bust through most zombies in two hits. I got up to 850~ kills pretty easily, but then the game started bugging out:
For one, I couldn't buy health packs once the cost reached 3150. I'd have enough money but the button would be reddened out.
Secondly, the zombies stopped spawning at 850 kills for me. Which I guess is pretty cool, since I could act like I "beat" the game, outliving the zombie onslaught. Cept I didn't get to enter my score, which is a shame.
I tried playing the game without the exploit, of course, but I didn't get very far. Maybe I just suck at jumping on zombies.
Ah well, still an awesome game. I look forward to seeing a number 2.