Mow Locust with Turret
Put the game on easy and go to the level with the drill rig. When you hit the lava field, a bunch of Locust will run at you. Hop on the chain gun and mow down the locust running out of the hole. Makes it really easy to get 50 kills in 30 seconds. Restart the checkpoint and you will be right in the same spot ready to kill the wave again. On easy, it makes it faster so you get them all in one pass before that big guy cuts you off. It still takes a long, long time to get the 100,000 kills but its faster than anything else and an hour of shooting gets you about 10,000 kills.