Like most of the enemies in the Heckran Cave, they can only be hit by magic attacks. Have each of them use their magic attacks and heal when necessary.
When Heckran says "Go ahead, try to attack", don't attack because a counterattack is to be expected. Use that time to heal up and wait for his counterattack to dissipate. After that, continue with the strategy.
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Heckran is a monster found in 1000 A.D. in Chrono Trigger. He is the "boss" enemy the player...