He learns a spell that makes life a lot easier. The healing spring right before you fight to Dhoulmagus and the forest west of the Royal Hunting Grounds are good places to level up.
What I did in the battle was: * The protagonist attacks, never psyches up past 20 * Yangus does Helm Splitter (six points in the Axe skill if you haven't used it) until it's ineffective, then just attacks, never psyching up past 20 * Angelo raises defense * Jessica raises defense or does Oomph/Accelerate * In the first part, focus on the clones
I had a monster team and two rune staffs (from the first casino) for the battle. Focus on the clones first.