He always attacks with the opposite damage type you use. If you hit him with Material damage, he will attack you with Force. If you attack him with Force, he will attack you with Material. (and IIRC if you don't attack at all for whatever reason, he uses his Stampede which is both damage types, but I only ever saw it happen once so I'm not sure.) Naturally, he will also change his defense type to match your attack type.
So pick one Material type (Nozomu or Nozomi are probably both fine), one Force type (probably Jatzieta), and one person with Icicle Arrow (probably Ruputna or Thalia). Switch their roles in attack and defense appropriately. Count his mana, and the turn before he drops Hydra, put up an Icicle Arrow (Ice Banisher isn't good enough to stop it at this point).
Jatzieta/Nozomi/Ruputna is a good team for taking him down. Here's a video of someone doing it with said team without taking any damage:
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