This achievement may not be as clean cut as it sounds. When you fall on another mech, you do very little damage. The easiest way to do this is wait on high ground until you see an enemy with low health or can manage to get someone to low health. Using RB to fly up higher will result in you doing more damage on the landing, but will make it harder to aim your landing location. This may be easiest with friends, having your friends lure enemies to vulnerable spots. This is all assuming you're against boosting, if you aren't, then it shouldn't be hard to find someone on True Achievements to boost with.
EDIT: I have since learned that you can also get this achievement against bots according to people commenting on my solution.