In the level "Martian Fashion Show" you will come to a Cloning room very near the beginning. In this area, run to the back to have all the enemies break out of the tubes, then stand on the crate in the middle so they all crowd around you. Now, spend a minute weakening the enemies with normal attacks and spin attacks () but try not to kill any. Once you feel they are weak enough, use a combo (or only if you collected all the tubes to upgrade your attack) and then ground pound (+) to kill the entire group.
Don't get frustrated, this will most likely take a few attempts (or a lot), but keep trying.