At first I had trouble with the boss but here's an easy way.

Go through the dugeon a couple times to get enough exp to make the battle even easier. But a city to the south called "The Village of the Ancients" contains all three level black magics as well as the white magic teleport.

I used three mages : 2 red (Luneth and Ingus) and a black (Arc). I made Refia a Monk. Have your red mages cast healing and ice spells. A monk can retaliate easily with the new claws found in the dungeon. If you keep the black mage casting ice spells as well you'll definetly win in a few rounds.

Another thing you should probably do is move them to the back, especially the black mage and the red mages too, if they have enough high level magic MP.