The Mortuary is my favorite level for sheer DOOM-style mad nonstop carnage. When I realized that changing targets during chainfire let you "hose around" with the bullet stream, the Mortuary became the most insane, psychotic fun you could imagine. With a bulk-modded Minigun it is joy incarnate. Full-out chainfire, hosing it around to wipe out six or seven formers/imps/etc a turn... striding knee-deep through the dead until you spot a Vile, and pumping him so full of lead in less then two seconds that his body goes CLANG when it falls?
Yeah. Oh, hell yeah. Felt like the Terminator.
The Mortuary is very hard, yes, but it's like any other Arch-Vile fight, only more so: to wit, clear out anything that's not a Vile by any means necessary, then feed the Vile a steady diet of Rockets. The trick is clearing a path to the Vile, which means you can't play whack-a-mole with their minions, which means you must deliver massive carnage to a wide area, immediately. BFG-9000 seems like the obvious choice there, but as observed, rapidfire builds with a decent 10mm weapon can saw through them wonderfully; if you know the "walking fire" trick. Double Shotgun or even Focused Double Shotgun can deliver similar rape, especially with Army of Darkness.
In short, you need a time-efficient snowplow. By the time enough monsters have been rezzed to worry you much, you've already sallied forth, found the Vile, and blew it to hell. Rinse and repeat as necessary. If you're a bad enough dude to use the vanilla rocket launcher, the blast radius of 4 makes it decent for that job, in addition to corpse gibbing - and you've already got it in hand when you spot the Vile.