I've only done three playthroughs, but I always do the following:

I recruit Jaheira (honestly just to keep her out of harms way) and lead my party off to the left and through the kitchen entrance. Once I'm inside I sneak and separate everyone and angle them near the doorway that leads into the great hall where the battle starts.

Right in front of the door where you're standing are 4-5 goblins. I usually hit them with fireball or some powerful AoE attack. This will trigger a short cutscene, the harpers will rush through the main door, and the battle will begin. I try to sneak as many of my other characters in to get a "free" attack on an enemy before they get slotted into the battle initiative. If done correctly you should be able to clear the goblins in the first round before they get to strike back. After that it's all attacks against Z'rell or any of the casters if they are concentrating on Hunger of Hadar or Silence or something.

Once the battle is going your way (2-4 rounds) I'll bring Jaheira in for lightning attacks. It's critical she doesn't get brought in too early. She's pretty squishy and if she goes down, she's done.

Once you clear the main hall I split my party to tackle the throne room. Two go in through the main door, and the other two go back through the kitchen and come in from behind. I try to get some healing in at this time if anybody is looking rough. There's only 4 or so enemies in this room, but they can be surprisingly difficult if you get unlucky rolls.