Strategy:City of Skulls

One BFG blast clear souls in big radius, so it is recommended to do it first thing after activating lever. If you for some reason can't use BFG, rocket jump away or use phase device.

After that, apply standard anti-pain elemental measures (i.e. shotguns).

Sufficiently strong melee also can work, as long as you don't have too many souls at your throat at the same time.

Start from one side, and advance gradually, so you are not attacked from different sides. Repeat for second wave. Retreat if necessary, to "compact" enemies for easy shotgunning.

Skulls that Agony elemental drops are useful for Mortuary, because they destroy all corpses at big radius. But most important is Hatred skull, because it is one of very few ways of getting berserk status on demand, and it lets even brute-less character to clear Unholy Cathedral.