Alien Hominid HD

On the mission where you first get your ship at the part where you destroy the chopper before the boss instead of getting in just keep head chomping people and practice making chains since the screen wont move and you get unlimited enemies just make sure to wacth out for the ones that shoot

The 2 places I found easiest is in 1-1 or 1-3 when all the agents are dropping out of the choppers. There are so many dropping down its easy to catch one without touching the ground and they will keep dropping as long as you dont proceed forward. Also doing this on easy doesnt hurt.