You’ll get holy magic and the ability to perform spell counters once you reach Ranbol. For the early game, the only way to flawless magic-using characters is with lunar magic and a whole lot of luck, so it’s not really worth it. The effects are largely minor anyway, and you can rematch them in the throne realm once you can counter their spells if you want.
To answer the question though, you can only be hit by tidal attacks and certain auras (wounds and poison, for example) and still get a flawless. You have to counter or deflect all attacks and spells.