Originally Posted by
Raistlin
My party was Lightning/Hope/Fang for this as well, and I didn't have any trouble with him. I think maybe you're focusing too much on weapons/items, instead of on battle tactics. The people that have had problems with that fight tend to be ones who, when Barthandelus is charging up his super attack (I forget the name), stop attacking and start buffing/healing to prepare. However, it's a much better tactic to keep on attacking hard, because the more you hurt him, the less damage the attack will do.
Also, one thing I did with Fang that was very helpful was level up her Ravager ability to learn Fire, so that I could attack with 3x Ravagers to build a quicker stagger.
LJ is right that you shouldn't bother too much with upgrading weapons (unless you're certain that's the weapon you want to stick with after having done your research). You're not going to have the money to do too much with it anyway until the end of chapter 11.