I'm going to attempt this without doing any of the cases. If I come up short I'll try doing it the case way.
Wish me luck!
I'll post what happens.
I'd recommend -WITHOUT CASES-. I managed to do 74.000 Kills as you can see on my picture in this thread. But some also managed to do it during the cases, guess of the more numerous Gas Zombies. Anywayz, let me know which method was better for you.