The Fire knight Great sword is actually a really good weapon, it does incredible damage for its weight. That is, if you're okay with the weapon being split damage. I would say it's even an upgrade over the Godslayer's.
I wish he would find the black knight weapons. They really suit his stats. The twinblade goes hard, especially with a buffing spirit art. But yeah that greatsword carried me through a majority of the dlc.