Zhalk and the Mindflayer give no xp, only the 2 cambions. Either way, totally unnecessary cheese as you'll reach the lv4 cap long before eating through EA.
It would be unnecessary, but I use the Level Up mod. I usually start a new game after reaching level 6 though. There are only 2 times in ea that I use barrelmancy. After my party jumps down the hole, into the ruins of the Chapel; I Firebolt the oil barrel, when I open the door. The 2nd is the cambions. I gain 3rd level before I leave the ruins. Not always though. I have a duel wielding strength based Hunter Ranger, that I'm not using the Level Up for. It messes up Gale's Flaming Sphere. The spell just set the ground on fire, no sphere. So 4th level will be enough on that playthrough.
