It's a slow and annoying process, but if you can get the Mantis in Big Team there's usually at least one person per game who will run right at you spraying an AR or something. The trick them is to resist the urge to blow them away before they get to you, or have one of your team mates do the same.
I've also heard that the number of stomps is inconsistent for unlocking the achievement, so that's why I keep score manually (because there's nothing in the game that differentiates between a stomp vs regular Mantis kill)
No, there's no medals for stomps. If you get the Wheelman ability, it makes it a lot easier. People will EMP you and think they have a larger window to board you than they actually do. Also, they need to board from behind, and you can rotate even while stomping. So if you miss a stomp, keep the front pointed at them. It'll make it a little harder for them.
"I figured maybe a bullet in the back. Or a knife in the back. Maybe hit by a train." "In the back?" "In the back, yeah."