You mean the same Demon Knight in Durlag's Tower, who appears when you first get there? I don't know, he was never much of a big issue to any of my parties I played. All party members were at the TotSC XP cap by that time, that's lvl 8-10 (depending on the class).
What the NPC's tell you is that there is the mirror behind the demon. So this is how I beat him, always without ANY casualities and without a bucketful of scrolls: I simply summon few mobs for distractions (scrolls and wands will do just nicely for that), get the attention of the Demon Knight with these mobs, while he's busy one of the chars cracks the mirror, some shadey figures come but heck, they aren't that big of the issue. Main thing is not to let the party to get separated, and attack all the same mob. Nothing is worse when your tanks get separated and each of them is fighting something on his/her own. If something then gets on the weaker party members, these will go down fast and your tank will be pretty wounded, if he/she will try to make his/her way to the party to save the weaker members. And will also then go down. Not to mention that from the broken mirror the stuff which comes out at times also helps you, in it's own way - the demon knight "clone" which came out fought the main demon knight, so it was even easier
Of course, I also play usually the tank char as my chars. I don't like men in dresses and so I don't play any such. I play on the Core difficulty rules, I don't want to spend more time on the game then I do by playing with the hardest of settings, and easier settings would be just too easy. Of course, as you noted I also did him with the party
I aint a big fan of soloing, since I personally feel that I lose some of the "shine" that way. To take him down solo, that's of course significally harder, and you did so, so grats, good work indeed!