Hit one or two of the key blocks with the hammer when the board is crossed. Then, eliminate almost all monsters, leaving two or three free. Hit the final key block on move to the top of the board, next to the door. Wait until the unicorns approach, collect the prize at the door, and use the hammer to finish the board.
Stuck unicorn kill:
Mr. Do can fall onto the last unicorn on the board if it is stuck in a hole and not lose a life.