I know how you feel!

Have you thought about applying special bonuses for 'special' people?
First Kill: Be the first to kill
Kill Joy: Kill
n number of enemies in a certain time
Blood Bath: Kill
n number of enemies (without enemies killing each other)
Sniper: Kill
n number of enemies within
x percentile.
Surviver: One on one win (meaning win level)
Hmm...
Also, weapon upgrades (can diminish over shots) throughout level... or weapon upgrade after levels (like a shop - time consuming though)
Sound effects would be great!