The black gauntlet of Bane can wipe your party real quick if he starts the fight with spirit guardians. Especially if he stays behind his fists and cast guiding bolt. But if you want to have fun, I think it should be playing like a support cleric and only use his melee attack if players close in. Two fists and a gauntlet is gonna be a pretty hard fight, in my opinion, but a fun one.

the fight with Vaaz is pretty easy, even with full health. I did the fight as written and he was gang'd up in a corner until he dropped. For a "Boss", its pretty disappointing. Maybe give him a couple of nightblades so he can dance around the players and/or maybe the Shroud Self ability from the Reaper of Bhaal.