That one is tough. The key here is hitting with the party members since they do not have reduced damage against the enemy. I heavily suggests you use at least 1 or 2 unique weapons since they add 2 levels to a party member. If you can manage to have a party member at level 9 this is the best because the hit rate of a party member at level 9 climbs at 6 for most of them. Then equip hit rate boosters for party members. Also, the critical ability of the Fighter is not affected by the resistance. So if you have a level 9 Fighter with 3 Fighter hit rate +, you will rake in the damage. It might still take some tries, but you'll get it eventually.