You first get the mission to go get the stingray after you undertake the mission "Intelligences of the Artificial Persuasion". Once you get it, you can either drive outside and find a bandit, or go to an area where you can find any enemies you can stomp on. Press LB to go into the air and then when you're at the peak of your jump, press RB to slam back down on the enemy. Just repeat until you kill an enemy and bam, achievement unlocked.
If there's any way i can improve this just let me know.