When you go down through the hole on the left side
to reach the monkey, quickly move over to the right side and destroy
the television to get a health ball. By this time, the Monkey
should be making its way out of the blueprints. Without shooting
at the Monkey, go back over to the left where the hole above you
is located. Since the Monkey moves around on the ceiling, when
he comes near, shoot at him. More often than not, that one shot
forces him down onto the ground where he moves back and forth
shooting. You should shoot too, and avoid his shots by jumping
up if all else fails. The Monkey will then jump back up and go
to the right of the screen. Stay there. Do not go after him, because
you will almost always get shot badly. Repeat the process until
he yells and bursts into pieces.