[Jack of Speed]

Ew, I didnt like that battle. If I remember correctly I beat it by checking to see who MOST of the delegation attacked first and slapping an Otilukes Resilient Sphere on them. Then I ran to the other side of the map with the stragglers that were still attacking the rest of the party killed them and went back and finished the others with area effect spells and summons... OH yeah... making yourself invisible BEFORE you go up the stairs didnt do anything because they can "see" you after they "talk" to you. BUT BUT BUT if you go up the stairs (looks like you got two wizards) get one to cast "Mass Invis" while the other puts an otilukes Resilient sphere on one member of your party. Make sure the sphere is finished casting before the invis is. but Sphere is a fast spell and should get off quickly. Then let everyone attack the "Sphered" character back off and pick them off with Summons, Ranged Weapons and Area Effect Spells. Im pretty sure this is how I beat them.
Hope it helps.