That assumes Celes is able to get a turn in between Terra starting to cast the spell and actually casting it. If all she's doing is waiting for Terra to act, then that means she has fewer chances to attack, so Runic isn't necessarily the huge advantage it seems to be at first glance.

Also, I presume we're assuming that characters can't equip Genji Glove+Offering, because if they can then whoever gets the first turn will annihilate the other two.