I just got this achievement after playing a few games starting from 0 trample kills and i would love to share with my TA community how i did it ..
I tried to do the kills for hours and hours in Walker Assault and Supremacy but never got close to an enemy before getting shred to pieces .. also the tokens for ATST where hard to spot or to get as all people who try to get this achievement are playing these gamemodes and are looking for it as well.
The best and by far easiest way is TurningPoint on Jakku (its a DLC you have to get .. no worries .. its free).
!! Credits to TheFourthHunter
for pointing out Turning Point as the best gamemode in the above solution's comments !!
When on the Imperial side (ATST's are imperial vehicles NOT rebel's
) you start with defending 3 points .. in this first "wave" no ATST tokens will spawn but after the first of those defend point is taken, you get up to 3 further "fallback" waves of 1 or 2 spots you have to defend and each fallback wave has its own ATST spawnlocation.
Below a very handy movie for showing the 3 locations .. check it and memorize the locations but after playing the gamemode a few times you'll find the spots quiet fast and easy.
Make sure to stay in that ATST spawn area and when the tokens spawn .. grab the ATST and start walking towards the defend spots because enemies will start to attack those defend spots .. if you die, just head back to the location where you picked up the ATST token as its keeps spawning at the same location each fallback wave ... I sometimes respawned next to it
The average amount of trample kills per match where between 8-10 .. It is cumulative so just play a couple of imperial rounds and .. Bob's your uncle
Hope you get the trample kills as quick as i did ..May the Force be with you ... always !!
Edit 1 : The pick-up token takes 20 seconds to respawn (roughly) after death. (thanx left 4 zaneage)