Rain of Blood + Bless. Then you can use the portal he activates when leaving by interacting with the Gargoyle above.
He knows that already, what he and many others I guess, are wondering about is where in the actual game (at that point) you can get a hint that blood douses hellfire. Not to mention that this is probably the only instance in the game that has hellfire, and what is even more confusing blood does not douse necrofire.
