for everyone having trouble with lvl 24, you need a base of 9000 points (3x gratuitous kills, 1x brains). it can be done by not maxing out the power when using slider missiles to take out the top 2 zombies. this way, the slider "misses" the head shot and pushes both zombies (also takes the brains for you) into the chainsaws. kill the super zombie by pushing it down the three slides.