Sorcerer, I have made three play throughs on normal (single player), having to start over and level more when I reach Diablo each time. The last stage seems to be filled with units that are immune to most of my spells. I have increased my spells a lot, so I hope this third run through is enough to kill Diablo.
Maybe if I took a run through at a higher difficulty I would have better gear and such, but it feels like cheating to not complete the story on a lower difficulty first.
Hellfire has been a Godsend, as it increases the loot and experience available (I never had the Hellfire expansion back in the 90s and 2000s, didn't even know it existed until I got it on GOG).
As for multiplayer, no, that's not my thing. I feel too old to put up with other people in games unless I know them well.
I tend to play warrior in that game, so naturally my least favorite thing down there winds up being the succubi. Not sure why the designers thought it was a good idea to have them fire bloodstar at the rate that they do when you can't move that fast? I guess it adds to the claustrophobic feeling.
On my last hellfire run I remember lightning wall helping me out a lot down there, that spell is nuts.