This monster is actually pretty easy. It'll run in a circle around the center of the room, occasionally stopping to launch an attack. That attack can be an attack with one or both of its claws, a spray of Defense Down causing bubbles, or its eye laser attack. The claws and bubbles are avoided by standing next to it. The lasers require actual dodging effort, and you'll basically have to get out in the open and rely on your reflexes and Haste to dodge them. If you're not running around the field to dodge the lasers, then you should be standing next to the crab and following it around, taking shots at every opportunity. It is recommended that the player focused on a specific target (claw is recommended) as the loss of one target will make this boss more vulnerable (taking more damage per shot).