Some finishes failed to count for achievement
Update:
Doing some more pvp today I’ve now managed to finish the achievement while only using 3 more stomp finishers (still have 12 left). It looks like if you cancel your own stomp, you still get credit. According to guildies this is apparently a known bug, so I’ll leave it at that for now. I may not have gotten credit for 2 of my stomps because I stomped someone I was never in combat with (did so in stealth after someone else downed them).