After you Enter, cast buffs (Make sure you cast Enthunder). Head to crabs. One rdm silences the middle crab, one binds the Left crab, one binds the middle crab. Kill the right crab, kill the left crab. One rdm casts slow, one casts paralyze, one casts blind. Kill the Large crab as quickly as possible and make sure to keep silence and enthunder up at all times. Don't worry about nuking, just keep hitting.
I've done it this way everytime and have had success. I believe the subs for the rdms were /whm, /blm, /nin.