Plug in thee other controllers to make it a 4 player game. Launch Forest 6 and use your main controller and go to town killing the required 350 kills. For whatever reason, the game counts your kills as if the other three got those kills as well. So if you got 20 before you died, with the other three it's like you technically killed a total of 80 enemies. I've had a few people confirm my method that it works.
This method will also work on getting the 150k crystals which is best done on Forest 5.