When entering aggro range of the enemies, Arciela will have initial hate from Balamor, and by extension Balamor's Sycophant as well, thus all the enemies will initally attack her, although upon engaging Balamor himself, players, trusts, and pets can draw hate, and will in-turn draw aggro away from Arciela. Regicidal Dullahan however, will always attack Arciela, regardless.
Balamor has approximately 62,000 HP (Dealing approximately 54,000 damage left him at 13%), meaning he will quit after taking 46,500 damage.
A suggested general strategy is to focus Balamor out of range of Arciela. Balamor will reign constant AoE damage on whomever is focusing either him or either Balamor's Sycophant. The Balamor's Sycophant have a lot of HP, probably near as much as Balamor, and aren't a huge threat until you focus them and build their TP.
Whether you can solo/duo/trio this one with trusts, depends on how well you can zerg Balamor before Arciela dies. She will stay alive a long time if you do this right, but it may depend on your party composition whether you send a DD after the Regicidal Dullahan.