Defeat The Horse Rider. He is the first boos and is found at the end of the first level.
When he is on the horse charge at him with any attack as he slides into you. As you do that you will damage him and he will loose the hits and not you.
In the second form just hack and slash till he is about half bar and B will pop up above him .. Hit him with the dirty finisher then Achievement unlocked.
VIDEO: Starting at around 5:40 Will show footage on the boss fight itself.