You must kill all mostriciattoli in the level, to kill them there are two ways:
1) Schiacciarli under a stone.
(2) Inflate them and make them explode.
For inflating them you will need to press SHIFT and then press the J or L depending on where you want pumping, Shift to remain down when you pump.

To start the game press the space bar if wrong click with the mouse on the image of the game and try again.

Use the keys to move:

The - K (up, Jun)
J l (left, right)
SHIFT + J or + K (pump)

Have fun...