The Dark One is not very difficult to kill. He usually carries the Eye of the Aethiopica and thus is magic resistant. Many newer players do not realize this and repeatedly cast spells like magic missile, which are ineffective.
His level allows teleport, so you can sneak attack him, grab the Eye of the Aethiopica after he teleports to the upstair, bag it, and teleport near to the upstair to allow you to unload with spells such as magic missile. Bagging the Eye is required because if you don't, he may steal it back and become magic resistant. He will place greater priority on healing than picking up the Eye, so if you can approach him without waking him and hit him hard, e.g. with a reflected magic missile, he will teleport to the upstairs and leave the Eye for you to pick up.
Another approach is to introduce The Dark One to a fast, tame purple worm, which will swallow him whole at hello.
This Quest nemesis can be easily dealt with, even without any spells. He does ignore Elbereth, but even that does not make him a huge threat (though he may be rather tricky to take out compared to most monsters you've faced so far). The most important thing, as before stated, is to hinder him from using the stairs. If one does not have the teleport intrisic and teleport control, it could be a bit difficult to hinder him from using the stairs. A scroll of earth can be very useful in that case. Read it after you get to this Level and move one stone on the stairs.