Mickey is the second boss in the game: a huge rat in the sewers.
Pay attention to the boss's schema, choose to move bottom when Mickey is jumping and viceversa. Don't forget to kill the small foes because they can be really painfull.
I've prepared a video, below: video is in 2 parts. During the first part I've unlocked the achievement but Michey killed me in the same time (gosh). The you can find how I beat Mickey and survived (*LOL*).