Thought it would be useful to share these tactics. No ADHW and no Zone of Sweet Air involved.
Those of the Rakshasa race seem to be immune to spells from levels 1 through 7 in Baldur's Gate 2. In NWN it's even worse, as they also become immune to level 8 spells.
Also, they're immune to normal weapons, and Ihtafeer himself requires a +2 weapon of some sort to hit.
Melf's Minute Meteors work (most of the time), and so does Abi-Dalzim's Horrid Wilting. Have scrolls of Zone of Sweet Air in hand (or memorized by your cleric or druid).
Your best bet is to have lots of Summons (the hard hitters are the Air Elemental and the Skeletal Warrior from Animate Dead). Pepper Ihtafeer from afar with either Melf's Meteors or some other +2 ranged weapon. Beware of Death Fog.
Solo generalist mage:
As a mid-ish level mage, you don't have access to level 8 spells or divine magic, but you probably have a few Summon spells and Polymorph Self. I felled Torgal in De'Arnise Keep with Summon spells, one Lower Resist, G. Malison and Slow. This tactic won't work on Rakshasa due to their aforementioned immunities (Lower Resistance won't work), but Polymorph Self is absurdly important.
Polymorph Self is the single most important spell in this encounter. Have it memorized along with the usual pre-buffs of your choice: Protection from Evil, Shield, Mirror Image, Stoneskin, Globe of Invulnerability. Pre-buff right outside the cottage, and don't forget to cast Polymorph Self (but don't transform into anything yet).
Step in, initiate dialog, listen to the short threat and WAIT for Ihtafeer and its bodyguards to cast their Cloud spells and Ice Storm. Quickly polymorph into a Mustard Jelly. Now you're immune to their spells, and their weapons can't hit you, either. They're doomed. WAIT for the Cloud spells and Ice Storm to end.
After their spells are finished (and gone to waste), polymorph back to your regular self and focus on your Summon spells: Skeletal Warriors work great. Spider Spawn works, too. Just have your Skeletons pop in first, as they're highly resistant to their weapons. You can add in an Elemental of your choice if you want. Because you're only using a few spells, fill the rest of your second level spells with Luck and bolster your Skeletons and Spiders to your heart's content. Game over.
TL;DR: Polymorph Self as M. Jelly to waste away their spells; focus on Summons afterward.