I dont think that monster summoning really helps a lot, coz Aec´Letec is pretty smart and doesnt use the gazes with summoned minions, only with party characters. Id suggest having a couple of tanks, equipped with potions of healing, fight the priests using melee, while the rest of the party uses arrows. Have at least 2 characters memorize dispel, so that they can dispel the gaze (for this reason its not worth using strenght, haste, or any other spells or potions in your party, they´ll wear off with the dispel). When you finish the priests, just go for the demon.
Other people likes using lots of fireballs, wands of fire, and arrows of detonating to kill the priests quickly.
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"Sigh" -Ertai, Captive of the Blinding Angel