The stomping is very inconsistent depending on the amount of lag. I have been stomp killed while behind and above the Mantis. It's easiest to get the achievement on Shatter since there are narrow pathways. Go in the rocky path next to your base, resist firing rockets, and stomp away while people rush at you. I got, like, 8 stomps in one game from idjits rushing me. Also, keep in mind that stomping vehicles will NOT count towards the achievement, as I found out the hard way.
"The most important thing is to never stop questioning." -Albert Einstein. Xbox Live: AlteraLateralus