This can be done against any of the main bosses in the game. This was recorded on Easy difficulty. Simply fight the boss until the become doomed (their health bar becomes a yellow and black bar). Once this happens, melee them and you will enter an execution animation for your achievement / trophy.
Here is a video guide with English commentary for this achievement!
Here is a playlist of guides for this game!
https://www.youtube.com/playlist?list=PL-HeW6XkKdUL1_8Q01lMy...