1. Destroy the marked green blocks.
2. Go up and unlock the red locks.
3. Take the yellow key to make the firebreathers fall down.

1. After the monsters have destroyed all purple blocks,
lock the red key locks again.
2. Take the cyan key. The bomb will fall down breaking the wood blocks.
Destroy the remaining green blocks, then the blue one.

Go up and release the firebreathers (yellow key).
Let the bomb fall down so it will be hit by a fireball immediately.
The explosion will break the mystery.