FormerlyHemlock
Hero
Just a quick note. This is one of the things that I've never liked about D&D. The obvious solution is to chop the Big Bad up into small pieces, box the pieces up in concrete (or whatever) and toss the pieces into the deepest parts of the ocean. Problem solved. (At least as far as they're concerned.) But you can't do that in D&D. Bummer.
Eh? Why not?
There's even a 9th level spell (Imprisonment) designed specifically for the case of a BBEG who just won't die. Not that you have to use the spell specifically--you can work something out at lower levels.
Amusing idea: a 20th level wizard with Int 7 and Cha 20 and Resilient Con and Lucky has a Magic Jar DC of 12 and +11 to Cha saves. He literally cannot fail to survive his own death until you destroy the Magic Jar. He does have a bit of a problem possessing anyone but a mook in the first place though... will need to be patient.
Normal wizards just rely on the Clone spell though, or turn themselves into liches.