go to the great wall of china with the pocket watch. Once there, just keep going to the right until you find a barel of gun powder. Then go back to the viking cave and use the explosives.
Go to poptropicasecrets.com they have video walkthroughs for all of the islands. It's too lengthy to explain here. However, I got stuck on the Hydra for the longest time and you have to step on its...