BG1 NPC related (dunnow if SCS too) --> the battle with Andarthe & his mate is pretty easy, just because he moves inside the tree house instead of casting spells. He then continues randomly moving in and outside the house. It's pretty easy to fight and kill both of them 'cause they don't attack always neither cast spell.
I got this too the first time I fought him. What he seem to do is move away from you to cast his spells. He is a coward after all. If you fight him too close to the tree house door he runs inside and not just away from you. I have fought him 5 or 6 times now and he only did it the first time.
mm75 you are posting in the right place for the mega mod, but when you have a good idea what mod has a problem (even if it is a conflict like SCS for BGT and BG1NPC and I am not saying this one is ) you can post in that mods forum and BG1NPC has only 1 forum because all versions (BGT & Tutu) are in one pack.