How did people beat him?
My party is level 19 and I've pretty much cleared Phantom Forest besides the Source Temple.
He casts four spells per turn and blesses his entire party. His minions are ridiculously good: fighters spam debuffs until one eventually sticks and then wrecks with physical attacks; mage spams damaging AoE fire spells which also heals the enemy. Their bodybuilding and willpower seem crazy high; it's pretty much a coin toss whether I can immobilize any of them with slow/blind/charm/knockdown. Their high HP means they'll recover before I can focus kill one.
Summons are so weak to be useless at this point. Accuracy of my fighter and marksman is 70-80% before bless/precision stance.
I don't even think you get exp for killing the minions.