Simply kill a total of 25000 enemies. This will come naturally through play but if you want to speed things up you have several options. First you can take a high level person into a level on Normal and just blast everything to bits. Second you could farm kills on the level "Ditchslap" by going into the tunnel with the three anthills and just keep killing ants as they pour out of the hills. Third you can (if you have if) use a pesticide weapon on any of the Chapter 3 levels. Without bots I racked up maybe 3-4 hundred kills a level.