I did this easy. i got 3 sharks and 3 blue dragons, put the 3 sharks up front and the 3 blue dragons in the back. As for spells, wait for 2 gems and use your most powerful water spell. I maxed them out and didn't have any trouble when going to the last stage :D
you may also need to have some good instincts on when to switch out sharks for others ;)
 "BoomsticK"){kind=icon}
 "Multitask"){kind=icon}
 "Treasure of Cutlass Reef"){kind=icon}
 "Brute Wars"){kind=icon}