During the Primary Quest "Emergency Situation", you'll get to a point where the objective is "TK throw clones back into source". There are an infinite number of clones spawning from a portal with no time limit or way to fail. They continuing spawning 9-at-a-time clustered together everytime the previous wave was cleared and aren't very strong at all, so there should be no danger to you, either.
This is an ideal spot to grind just about any power or weapon kill up to this point. For this particular achievement, just stand near where they spawn, let them do so, stomp, repeat. This is by far the most efficient method I was able to find.
A word of warning: as of when I did it at release, the game doesn't like to count Stomp kills very often no matter where you do them. So, just keep at it. It'll unlock sooner or later (as you can see, I do indeed have it).