I have fought the two Tanar'ri and that other demon with at least four different parties. Sometimes (once actually) I rock them, but usually it is a tough fight involving reloads. Here is my recipe. Buff with the cleric spell free action, any potions you can get your hands on and go on a summoning frenzy before engaging. Also, do not forget resist fear. I do not remember if the Tanar'ri can cast fear, but the one in the middle of the map sure can. Demon fear can turn a good plan into a disaster
. Fire elementals are good if you have them, but this quest is often done early in the game.
Concentrate all attacks on the demon and let your summoned cannon fodder deal with the imps or mephits. An alternate way is to take them down first and have everyone pile on the demon. This is one of the better places to let loose with wands of lightning as you do not have to worry about ricochets.
One of the best battles I ever had in BG2 was here. It was right up there with the T"§$%&d R"§e. My potion buffed Cavalier and Amonen had boosted to 23/24 STR, stoneskin, etc. Our summoned pets were Fire elementals and skeletons. Our little army charged and took down the first group. PC and Anomen hacking away, Val on the bow dealing with imps, J on the sling doing likewise and Aerie/Nalia really going at the demon with wands of lightning. Afterwards, the beasties saw imps, salamanders, whatever in the fog of war and took off after them. We followed and it was a running battle across the whole board. MASS MELE, WHOOHOO! It was almost like playing one of those real time strategy games. We cleared the whole board in about 1/2 hour of real time (including paused head scratching time). Usually I am content with a single demon heart and I drag my battered party back into the sphere.
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- Cayle