After being beaten by him several times, I discovered by accident that you can have 3 of your four characters out of combat.
So what I did was place my ranger in the middle of one of the grates before Boreas. Then I place one of each of the other party members next to an elemental pillar. I started dialogue by having Madora strike her pillar, this started combta but only with my ranger. So I clicked back to Madora (thus freezing combat), destroyed the pillar she was at, and then moved her into combat. After that I switched to each of the other characters and destroyed their respective pillars, and in the end I had all four pillars destroyed before even starting the first combat round.
After this it was all about having 2-3 summons out (I had a constant undead warrior, ice elemental and fire elemental out) to take the insane damage from his adds and to take them down fast. Basically it was a simple by then and I had him down before he got to call his Air Elementals to the fight.
Hope this helps. It is a really difficult fight, but figuring this strategy out and then owning him was really fun. I enjoyed it.
