It's a super-hectic battle at the end of Mission 12 with tons of tough enemies in super-tight quarters downstairs and on narrow pathways upstairs, with numerous pits you can fall into full of electricity.
The battle starts off with two Barons, although you might luck out and only one will approach you. There are at least four Pinkies (maybe six?), and at least six Mancubi (maybe eight?). Half of the Mancubi are Cybers. There are tons of Revenants, and a number of Cacodemons. Heck, there are even Lost Souls. But mercifully, no Hell Knights. Those guys are manageable in other battles but would've been too much here. Granted, this is all just on Ultra Violence; I'm not sure what Nightmare throws at you.
Depending on RNG, it could be a bit easier. And actually, the second time I played it (for fun) went a lot more smoothly because I used six BFG shots (three I brought with me, three more I found in the arena two downstairs, one upstairs). I actually misfired one, but the other five were enough. Also, if you have the "second chance" rune, you can trivialize it. And the Super Shotgun with Double Tap also helps.