The most important thing is to buy Flare Capes, since nearly all his spells belong to the fire element.
Use Kyle and try to push him to the right. Nanaly and Reala should attack with Splash, since Splash is most likely their strongest attack right now. Reala should also heal, since this will be important.
The crab will cast Fireball and Volcanic Ray. Volcanic Ray will be used against your casters. But you can still interrupt him because the casting time of V.R gives you enough room to attack him.
He has, of course, access to Spirits Blaster. Pray that he will not use Volcanic Ray often.
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