The boss on level 500 can only be damaged by healing.
This means that you cant use the Auto mode for this because the AI doesnt fare well with healing the enemy to damage it. They just dont understand it.
The way i beat this boss is by using 2 knights with the Dhronen weapon. And 3 healers to actually fight the boss.
The Dhronen weapon will put the enemies to sleep. This is nice to buy extra time. And then only the boss will attack you.
While you keep the others busy, you just need to keep on using your healers to fight the boss, and heal low health allies.
Perhaps also take some MP gain with you incase it takes too long, so you dont run out of MP and die after all the effort.