The easiest way to win this fight is to use a sac-pull method to only pull the Boss monsters, ignoring and de-aggroing the weaker monsters. Monsters have Alliance hate only, so make sure the pullers are outside the alliance before beginning.
Only the boss needs to be defeated in order to win. Doing so will instantly kill all other monsters.
Cyanic Crabs are resistant to magical damage and cast Water III.
Killing the crabs cause more to repop from the starting area. This is untrue - other crabs from deeper in the battlefield will come to the defeated crab's corpse. it can be one easy crab, 2, 2 and the boss, it's really random. make sure you are at least 16' from the corpse and you will not be aggrod. the crabs come to the corpse and will stay there, they will not move around. a strategy that i've been developing is to use this to your advantage and pull the crab that comes to the corpse if it is an easy one. i also make sure to kill the crabs along the walls and plan an "escape route" of where i will go next to avoid aggro if it does happen to be the boss that arrives. this can also be a strategy to easily fight the boss without having to Sac pull.
Upon killing crabs, occasionally the boss Caldera Crab will come to its aid, running to where the crab was killed.