Many of his spells will be ineffective against a wizard with magic resistance. Although the Eye of the Aethiopica confers magic resistance when worn, monsters will not wear amulets of ESP, so you are free to use wands of death or the finger of death spell on him. However, his high monster MR makes him likely to resist other effects such as sleep rays or polymorph beams.
Like most covetous monsters, The Dark One should be fought from the upstairs, but preventing him from doing so can be a bit difficult without a means of controlled teleporting. A scroll of earth can be very useful in that case - read it after you get to this level and move one boulder onto the stairs. Another possibility is to teleport and take his place on the upstairs.
If generated at level 16 or higher, the Dark One will be able to summon nasties, so if this is likely have a plan for crowd control or even escape ready.
In NetHack 3.4.3 and earlier versions, as well as variants based on those versions, carrying the Eye was enough to grant magic resistance; this makes magic missile and the finger of death (or wand of death) completely ineffective.
The Dark One tends to place greater priority on healing than picking up the Eye, so a wizard that can approach him stealthily and hit him hard (e.g. with a reflected magic missile) can force him to warp to the upstairs and leave the Eye of the Aethiopica free to take. Make sure to stash it in a bag to prevent him stealing it back. His level also allows teleporting, so you can also sneak attack him and grab the Eye after he teleports, then either teleport near the upstair to unload spells on him - or else simply zap him with a wand or finger of death.
The encyclopedia entry implies that the Dark One is in fact Sauron, the chief antagonist of Lord of the Rings.
... But he ruled rather by force and fear, if they might
avail; and those who perceived his shadow spreading over the
world called him the Dark Lord and named him the Enemy; and
he gathered again under his government all the evil things of
the days of Morgoth that remained on earth or beneath it,
and the Orcs were at his command and multiplied like flies.
Thus the Black Years began ...