To make an enemy fall, you need to jump-kick them when they are near the a ledge. There are plenty of chances to get this throughout the game. If you do not, a good way to boost it is by playing Level 5-1 a few times. There are 4 or 5 opportunities to make enemies fall in this level. Make sure you finish the entire level and do not just kill them then die, it will not save your progress. This achievement has been reported to be glitchy by some and does not unlock after making 10 enemies fall and requires more falls or a complete game uninstall and reattempt to unlock.