All of the Cathedral (except for The Butcher) is crawling with Undead. Basically, you will have a chance for killing some of them by Fire, Lightning or Magic but mainly on Purgatory mode. On Doom mode they're all triple or quadra immune and basically only affected by Holy Bolts. On Horror mode you also use Holy Bolt as your main spell there. Simply because it affects them all and it has decent damage (especially, at the very beginning).
Note: every area consists of 4 levels in the game. And every last level of the area is a hard one. Thus level 4 is the gathering of the worst forces of Cathedral. No wonder you got difficulties there. Actually, the game is NOT meant to be beaten without restarts. It CAN but that is VERY hard you know. It also depends on how lucky you get with all the drops, monster and boss placing, and dungeon layouts.
The damage you do with Holy Bolt does increase with your levelups and your Magic stat. Not quickly though. And that's what makes you face a great challenge on floor 4. It is supposed to be hard. But it is very well doable mind you. Many have done the area successfully. And that is because of keeping on experimenting and trying different things.
As a Mage you might want to master setting fire or lightning walls to stop them from approaching you. Or you might just lure them one at a time. Maybe lure them to a doorway so that they won't attack you in groups. Experiment to find your best tactics.
The Hell is the hardest of all HellFire mods. Don't expect it to be easy. It allows you to build some ultra powerful characters, but the opposition is also very tough.
Last note: cathedral contains undead, but later you'll have to deal with animals and demons, which means you'll have to switch tactics and elemental damage fast. The only other area where you come across Undead is Crypt level 1, but the whole Crypt is a one big Uber area. Those brutes there show no mercy.