Instructions
Kill Bulnoil's two guardians, Olven and Gavels, before you start attacking him. Take out Olven first because she is a ranged fighter and her attacks are quite strong. Get in close to nullify her ranged attacks and force her into close combat.
Ignore the creatures under the guardians' control. They are all fairly strong, particularly Lilith and Lucifer, but once you take out a guardian, the creatures stop moving and attacking.
Kill Gavel once you have finished defeating Olven. Although he is not a ranged fighter, he still has powerful attacks as a Death Knight, so advance on him with caution. If you have ranged fighters, use them to attack Gavel from a distance.
Avoid surrounding Bulnoil with your characters once you have defeated Gavel. If you surround him, he will stop using Meteor Doom and start using Geno Thunder, which is quite a devastating attack. Keep your party's HP up using any healers with the Area Heal ability. Keep your healers as close to the center of large groups as you can to make Area Heal more effective.
Attack Bulnoil to break through his barrier. It starts out as a purple orb, then fades to red as you attack him. Once you have damaged the barrier enough, it will disappear completely. Launch a full-on attack when the barrier is down. When Bulnoil's HP is down to zero, the battle is over.