Heckran's defense is quite high, so defeat him with magical damage of any or no element.

Heckran attacks with Water-based magic and occasionally his claws. He will begin using single-target Water spells for low-to-moderate damage and then do some single-target Cyclones for moderate-to-high damage. Watch your characters' HP, as early in the game most characters may not survive two of his attacks back-to-back.

When Heckran goes into his defensive stance (“Go ahead! Try and attack!”), attacking him will trigger a powerful party-wide Water 2 spell that is best avoided. Use this time to heal up if you need to. Watch for the battle message “Brief counterattack break.” Once this pops up, you're good to attack him again, even if he doesn't appear to have disengaged from his stance yet.