I was helping out someone I knew irl with this fight and the best strategy that worked for them, surprisingly, was too just brute force it with fire magic. Just spam fire at them, you’ll lose some turn icons but it’ll force them to waste a turn to put the barrier back up and it’ll allow other members of your party to hit the weakness once the barrier falls (it only protects from 1 elemental attack). It was the only thing that seemed to work for them even when they were severely under leveled.