The bombs have to drop down in this exact order.
First 1, then 2, then 3.

Remove purple blocks as shown in the image.
Then release the firebreathers.
They will drop down and their explosions will
open the passage to the left (at position A).

After having opened the passage to the left
try to go to this position.
Watch out for fireballs!
Then drop down the firebreather so that he
will shoot at the mystery block.