This was a pain but what I did was abuse the back-kick(
)plain and simple, What your going to want to do is stand facing away from the enemies and wait until they get close, maybe an arms distance away and just kick, It's a 1-hit kill on all small enemies and 2 on larger guys. Keep all of your Specials(
) for the bosses and use all the pick-up weapons on groups. You need to beat the 2nd level boss to get to 30K, It took me a good 10 tries to finally get it so don't feel disheartened if you fail a few times, All the enemies appear at the exact same spot every time so it's just memorisation. EDIT - Quanticjustice had added you can press RT to perform the same move!