Ashikaga Takauji, @, is the Samurai quest nemesis. He wields the Tsurugi of Muramasa against you, and can therefore kill in one hit. This, along with the fact that he ignores Elbereth and can teleport, makes him one of the most dangerous quest nemeses, though his attacks other than the Tsurugi are unexceptional.
Consider leaving Ashikaga Takauji alone until you actually need the Bell of Opening, since neither the Bell nor The Tsurugi of Muramasa are needed in the mid-game. Once you return with a full ascension kit, you'll be much more powerful, more accurate and have more options to deal with him. The remainder of the quest is still worth doing for high volume of loot that can be found.
The easiest way to kill Ashikaga Takauji is to zap him with a wand of death, which you may be able to obtain from a monster on the quest (attack wands are plentiful).
A wand of sleep may also work but carries the risk that he will resist (about 40%) or wake up, and swing the Tsurugi at you. If you take this route, it is strongly recommended to wear an amulet of lifesaving before engaging, to reduce the risk of YASD to an unlucky Tsurugi hit.
An alternative strategy is to polymorph into a large monster to avoid the risk of being bisected. Polymorphing into a black dragon adds the ability to kill Ashikaga Takauji instantly with disintegration breath via the #monster command. Neither the Tsurugi nor the Bell of Opening will be harmed.
It is very easy to approach Ashikaga Takauji without waking him due to the layout of the level; the final entrance before the inner section gives you a clear shot with any ranged weapon while still being far enough that he will not wake. He has no elemental immunities so an attack wand that hits will do enough damage to cause him to immediately flee to the up staircase, ignoring the Tsurugi. You can use this opportunity to pick up and stash the Tsurugi before he can return, making him relatively trivial to defeat. Alternatively a powerful pet could be left at the upstairs to defeat him as he tries to heal.
Ashikaga Takauji was an historical figure of medieval Japan. He was Shogun from 1338 to 1358.
Ashikaga Takauji was a daimyo of the Minamoto clan who
joined forces with the Go-Daigo to defeat the Hojo armies.
Later when Go-Daigo attempted to reduce the powers of the
samurai clans he rebelled against him. He defeated Go-
Daigo and established the emperor Komyo on the throne.
Go-Daigo eventually escaped and established another
government in the town of Yoshino. This period of dual
governments was known as the Nambokucho.
[ Samurai - The Story of a Warrior Tradition, by Cook ]
On July 8, 1336, he and his samurai entered Kyoto, forced Go-Daigo
to retire, after which he seized power himself and installed a
puppet prince on the throne (the current Japanese imperial family
are the descendants of this puppet emperor that Ashikaga
Go-Daigo escaped, though, wouldn't admit to have been defeated,
and opened a new "capital" in Yoshino (south of Kyoto), where he
and few of his descendants claimed to be running a government,
known as the "Southern Court". The period between 1337 and 1392,
when Japan was ruled by two courts, is known as the Nambokucho.
That "government" disappeared, naturally, after a few generations,
and Ashikaga's Muromachi regime lasted for a long time. The last
Ashikaga daimyo in power was the 14th descendant of Takauji.