A very good place to work on this is in the second mission of the dominatrix dlc.
Right from the start you have to fight her and then an endless amount of gimps comes in waves to fight you and they count as alians.
Its also right in front of a friendly fire so you can swap and upgrade guns if needed.
Be sure to fly over to pierce to get a checkpoint before aborting the mission or your progress will be lost. You should also take notes of what you still need because you cant check challenges while in the dlc.