MP is hard as heck without cheating if I recall. ESP if you're on hell difficulty.
I beat diablo with a warrior on mplayer years ago and I recall it being very close with the best gear you could have, when you couldn't enter a game without someone handing out the set of obsidian rings, godly plate of the whale, stormshield, kings sword of haste and whatever helm (probably veil of steel).
I found big D rather easy with a sorceror on sp (normal difficulty, as i didn't have a bnet char to change difficulty...) used firewall extensively on the mobs at lvl 14 and 16, chain lightning at lvl 15 (didn't get fireball... :/) and holy bolt on diablo himself, wich stunned him, so it was just keep firing and drink mana pots till he dropped, with him constantly stunned (i was level 26 i beleave) :P
With the mp sorc i just started it's considerably harder(okay, he's beyond naked. so what :P) I died at level 5 to a unique, then died 16 times trying to recover gear, and then decided to stop... too bad, as i just found a nice ring with -10 magic :P