Creepers are the bugs that crawl up to you and try to melee you all the time. To perform a melee finisher it is much easier to stand still and let the creepers attack you and then press to finish them. You could also wait from them to spawn out of a pod and melee them as soon as they come out. A melee finisher looks like you stomp on their heads instead of just punching them. To check your progress press > Extras > Player Stats > >Achievements and then find the achievement and it will have your current progress.