I'm assuming you are referring to the Second Floor of the Lair where you have to go through a shielded staircase. The place where you fight his right-hand man and several other Firegiants.
Other areas only have 2 Firegiants at a time.
Some interesting notes.
There are only two entrances, note that the entrances are too small for the Firegiants to go through.
Firegiants are not magic users(except for his righthand man), and
NO way to detect invisibility.
What did I do?
Used one mage.
Get ready either use Staff of Magi, or Potions of Invisibilty. Wear Boots of Speed too.
RUN through the entrance and attract all the Firegiants. When satisfied, go invisible.
While they are still blur as to where you are, cast Misled.
The cute thing is, since you were originally invisible, your image is invisible too, making you... well... totally invisible. 
Since you can cast offensive spells invisibly under Misled.
Cast Simulacrum if you can, not necessary, but it can help. Cast as many Greater Malison as you like at the Fire Giants. Two is usually enough.
Here's the kicker, cast
Emotion.
The bigger they are, the ~~ ~~
Ding! Ding! They are knocked out cold. Rush in the rest of your party and proceed to kick their butts to kingdom come.
Hope this helps!
Using this tactic, I did not even get a scratch. Joy.
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If not a Sorcerer, then a Paladin.
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Baldur's Gate Item Mart