If the above does not work I found a much easier spot to do this was level 7. When you are trying to find the conch sisters, instead of doing that just fly around and gather a group of enemies following you. When you are flying over the water no attacks can damage you so use this chance to shoot until one of the krabs drops a blue orb. Then hold LT until it explodes. Fly back over the beach to get another group, lather rinse repeat. It should only take a few moments because you will usually have 5 or so explode.