For those of you who loath XP, here is how I (mostly) avoided getting it while completing the Firewine ruins:
* Obviously, one shouldn't give the old armour to the six ghosts, as that alone grants 1500 XP
* When approaching Lendarn and the Ogre mage, I equipped Imoen with Talo's Gift (boots with 50% electrical resistance) and Batalista's Passport (ring with 40% fire resistance) and had her drink a potion of 50% fire resistance and a potion of 50% electrical resistance (can't remember the name). Lendarn is quite fond of the lightning bolt spell and both him and the Ogre mage loves the fire arrow spell.
I had Imoen run straight past Lendarn towards the Ogre mage, pulling all the minions with her on the way. Shortly after Imoen reached the Ogre mage, Lendarn's first Lightning Bolt went off and caused a carnage in the Ogre mage's room. Though injured, the Ogrillions still managed to hit Imoen twice, bringing her down to half health (12HP). At this point Imoen had already been confused by the Ogre mage, but she held her position as a target for Lendarn's second Lightning Bolt. This killed the Ogre Mage and left only a single injured Ogrillion and an uninjured Kobold shooting arrows at her.
Imoen, still confused, did the right thing: attacked the remaining Ogrillion, which she (eventually..) managed to kill!
In the meantime I sent my main troops in to deal with Lendarn. As they all have really bad THAC0, and Lendarn had cast Improved Invisibility, nothing much really happened from that. Garrick hit once with Chill Touch, while Jaheira suffered his Fire Arrow spells. I had my charname Enchanter ('Eclatan') cast charm person on the remaining Kobold, which I then sent to attack Lendarn as well. After a short while, the Kobold managed a hit, and Lendarn lay dead by its feet. As a thank for its heroic deed, I surrounded it and pelted it with blunt blows on its head.
Thus I ended up only getting XP for one Ogrillion and one Kobold in the boss fight. Neat! When Lendarn fell, Imoen was already back to her old self and I went to Mr. Luckyfoot to collect my reward, which, thankfully:
* Only consisted of 250 GP! No XP!
At this point I still had one Charm Person spell memorised. Encouraged by my success, I felt like trying out how much this spell could do for me against the 4 mercenaries on the lower edge of the map. Eclatan went forward and immediately cast the spell, successfully, on their fighter. She watched in awe as a plethora of spells way beyond her ability was unleashed on him by his former comrades. When things came quiet, the fighter stood Enfeebled and near death.
Eclatan poked around in her scroll cage to see if she could find some extra fun with the Charm Person spell. She found two scrolls! After using those, and a potion of Invisibility, both their fighter and their priest lay dead. She decided to let the two gnomish types live, so all she had to do was loot the bodies. Unfortunately, looting dispels Invisibility (in v1.3) so she quickly had the two gnomes on her tail. While dodging them, she also concocted a following of two Dire Wolves. Narrowly avoiding death by jaws, she made it back to the rest of the party, which joined her wild panic run towards the western edge of the map. To complete the hostile entourage, a Ghast showed up, snapping after both Jaheira and Dynaheir. Luckily both attacks missed and the fearfull heroes could escape back to Beregost with the mighty loot: a chain mail +2.
Unfortunately they're not allowed to use chain mails until after the bandit camp. Oh well.., now at least it's ready