Ashikaga Takauji, @, is the Samurai quest nemesis. He wields the Tsurugi of Muramasa, and can therefore kill you in one hit. This, along with the fact that he ignores Elbereth, makes him one of the most dangerous quest nemeses, though his attacks other than the Tsurugi are unexceptional.
The easiest way to kill him 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.