Master of Thieves
He is really no trouble at all to a player who is strong enough to fight through the crowds of soldiers that defend his lair. The most important thing to note is that he will pick up the Platinum Yendorian Express Card as soon as the player disturbs him, and will therefore have magic resistance--so death rays and so forth will not hurt him. This is scarcely a problem, as he is relatively unimpressive in melee and lacks special attacks beyond being able to steal the PYEC back from you if you get it before killing him.
Interestingly, just as he can steal the PYEC from a tourist, he can also steal the Master Key of Thievery (or any other covetable artifact) from a rogue. Of course, attacking him will usually result in your instant expulsion from the quest branch, but killing him in one hit (e.g. with a wand of death) won't. If he then leaves a corpse, you can remove it and revive it, if you really want to see him steal the Key.
There was a flutter of wings at the window. Ymor shifted his
bulk out of the chair and crossed the room, coming back with
a large raven. After he'd unfastened the message capsule from
its leg it flew up to join its fellows lurking among the
rafters. Withel regarded it without love. Ymor's ravens were
notoriously loyal to their master, to the extent that Withel's
one attempt to promote himself to the rank of greatest thief
in Ankh-Morpork had cost their master's right hand man his
left eye. But not his life, however. Ymor never grudged a
man his ambitions.
[ The Colour of Magic, by Terry Pratchett ]