The description for this halberd says it can kill outright Fire Elementals, Salamanders, and Efreeti. How about against Fire Giants? Does it do any extra damage to them?

well, the desciption does not say that it does, and the only extra damage that it will cause is that water drain(extra 15 pts of damage).

I used it during the whole Yaga-Shura ordeal, any extra damage you can get on those fire giants is good.