Upon beating the game for the first time, you will unlock a new game mode called campaign remix. In this mode, the game spawns almost all types of enemies in a given level instead of just the ones you see in the normal capaign. Although the spawns seem some what random, the best place I found to grind this out is on chapter 1, part 4: "Ditch Slap". Once you clear out the drainage ditch, you will have to plant explosives on a blockade in order to progress. Then you will enter a tunnel with three anthills. On campaign remix, you have about a 50/50 chance that the game will spawn spider nests instead of anthills. If you get anthills just restart the level. Using this method took me around 1 1/2 hors to grind out my remaining 7000 kills.