You fight this boss on level 10 after the level "Mind The Gap". The rat will chase you while you speed away on a subway train. Attack his legs first to make him stumble and get further away from you. After doing this a couple times hit the notes by his head to attack him and hurt him. When he sends his rat minions at you just kill them quickly to build your health back up. Rinse and repeat.Click here for tips on the next boss.