Divine weapons stop the skeletons in the catacombs from reviving when there's a necromancer nearby.
It's cursed weapons (or having a curse, or pseudo-curse via a Curse Transient item) that let you hit ghosts.
You can find 2 transient curses in New Londo before encountering any ghosts at all. They last about 5-10 minutes I think. Ghosts drop Jagged Cursed Blades which will let you hit them whether you're cursed or not.
Bring the serpents ring that gives you increased item find, and you'll find a blade in no time.
Also, ghosts often drop 2x Transient Curses as well, especially with the item find ring equipped. It really wasn't as tedious as it sounded. I never ran out and didn't feel the urge to hold onto them and not use them.
Yep, ghosts dropping transient curses made me way less wary of my use, since I tend to overuse necessary items in games. But luckily, even if you run out of transient curses and can't find the jagged cursed blade, you can always go into the depths and actually curse yourself via the frogs.
It's sort of permanent, by which I mean you can buy items that reverse it, but otherwise it sticks with you.
But in the end, it's actually worth it for your trip through the ghost-infested ruins.