I swear, officer, I haven't been drinking...
Well, except for the blood of that hobo that we in the Ancient and Pussiant Order of Dagon sacrificed to please our Dark Lord and Master and give Him more Power so that He may speed the coming of the Day of Destruction.
*bzzzzzzZZZZZ-WHAP!!!!
And now Way Too Much Time, Ltd. brings you the Unkillable NPC. While not technically a smackdown, this will annoy the party to death.
You need an NPC capable of casting Genesis and Astral Projection, and who perferably will live for quite some time. He casts Genesis, creating a pocket plane that he then covers with the most power protection spells in existance. Nondection, Misdirection, Dimensional Anchor, Unhallow, and as many Glyphs of Warding, Symbols, Sepia Snake Sigals, traps loaded with Power Word: Kill and Mordenkainen's Disjuncture etc as he can. Then load up your private paradise with all of the equipment and supplies you need for centuries and run your Prime Material holdings through an Astral Projection. If you ever get killed, come back and get the people who killed you.
The party will be very annoyed after they 'kill' Dragunibanipal the Great Red Wyrm Wizard 20 for the 5th time in a row without getting XP or treasure, Hehehehehe.
And for that extra bit of evil, cast Hide Life, encase your finger in Adamantine loaded down with really high powered protection and misdirection spells and feed it to the Tarrasque. Now anybody who manages to find your pocket plane and survive the thousands of insta-death traps to kill you will just get a very nasty and personal surprise a short while later.