The goblins vanquish a monster in the tower and receive a slingshot in return.
How to Clear Gobliiins Level 21
Have 
Dwayne/Oups use the slingshot on the bananas to knock
them down. Then have 
Bobo/Asgard punch the lever to the right
of where the bananas were. Have all three goblins walk onto the shark that
appeared in the moat.
This completes the level. Make a note of the code in the bottom-left of the screen ( ), then click anywhere to continue to the next level, The Giant Bird Skeleton.