What level are you? I didn't find that fight particularly hard (tactician), but I was level 9. Levels do make a difference in this game.
Did you aid the gnolls from the charm lady by breaking the charm? If so they enter the fight to help you too. I just opened the fight with AE, and other than my fighter (Karlach) they all just stayed on the stairs and AEd everyone (keeping the AE away from the harpers were the hard part). Ice storm works well since it causes enemies to fall (and many harpers). I had to reload the first time I tried because Jaheira ran into someone and got aggro from everyone for some reason and died as they all went for her, but when I reloaded she didn't get aggro and everything was good.
Open fireball(s). Ice storm. Blade storm. Shadowheart with the shield pet and AE shield. Speed potion or haste on Karlack and she goes on a rampage. Fireball + Karlach oneshot the ogre. Maybe try looking at your consumables like bottles / elixirs / scrolls to see if you have any damage / buffs that would help.
EDIT: Forgot, counterspell is really good in that fight, as with most others. I had it on both Gale and my bard, so other than cantrips, most spells were blocked. /edit
PS: When you do finish that fight, remember to enter the prison again, I almost missed that.
Where are the gnolls? I don't think I did that part. I'm level 8 currently.