Monk won't do too well. I reccomend making that monk a red mage, so you can have two healers if you need to. Use Arctic Wind items and Blizzard spells, get ice equipment, and fight it with two swords instead of a shield, one of which being ice. Use Blizzaga, then Blizzara. Always cast Cure on the whole party, no matter what, and Cura if you need it. Make sure the mages are well-equipped also. Other than that, keep trying. Lv 19 might be better, but I did it at lv 17-18 the five or so times I've played the game.