After a few rounds of pvp and stomping numerous players I was surprised to see that I still hadn’t finished the achievement “A Stomping Good Time”. I already had a hunch that this was because I was playing thief and using Impact Strike so before entering the next match I took a look at how many Mad King finishers I had, how many stomps were already recorded and then compared the numbers after the match. Result: finishers were being consumed by both impact strike and normal stomps but only normal stomps count towards the achievement.
Seeing as the finishers are being consumed when using Impact Strike I really think it should count towards the achievement as well.