Ok, so I used the pick axe with the monster (pressing Space) and then they spawn slimes
With the slimes I bougth a bomb and the watering can.
I could remove the boulders with the bomb, and get a bag full of money.
I used the watering can in almost all places, and the watering can but couldn't figure it out how to continue.
The game is nice, I like a lot the bg song in the hell room.