In a game trying for the lowest possible character level finish, Ariana did the blessing ceremony at level 12, so was level 17 at the start of the wastelands. This involved a lot of saving and reloading to get through the required fights (like the succubus), so she quit at that point. She used a survivor, for the sneak ability, though a warrior has the advantage in damage bonus per strength point and hit points per constitution point, and can use shadow potions and Embrace Shadows.
Most of the combat in the wastelands is optional. I cleared the initial area, around the town and went over to Patriarch, but after that got bored with the combat and simply ran past most of the opponents in the rest of the wastelands and Black Ring dungeon.
Iona's dungeon and the treasure room can be done without much combat. In the treasure room area there are 3 shadow potions before the group of opponents, so you can get by, talk to Zandalor, visit the vault and return (though if you are too low you might have a problem with damage from traps there; SpellShield would help). Lower levels in Iona's dungeon are optional, and after you get your equipment back you just need to head west to the exit; there is one door on the way you need to use a lever to open (which would turn you visible), but you can bypass most of the combat.
Reloading to get Frost on a weapon would help with combat, if you wouldn't consider that cheating, or you could use
DAD's item editor or the
Divine Divinity Item Editor to give a weapon a large damage boost.