I'm currently in the first mist generator castle where Zeto is. How do I beat him easily, as the call wave is very powerful. I'm currently at level 12.
Here's a video that shows a strategy for level 12 players:
The basic strategy appears to be to focus on defense when Zeto uses Call Wave, as that indicates he'll be doing a powerful attack the next round. After this attack, focus on healing first before you return to dealing damage.
This FAQ suggests the following combos to deal damage: