Numion said:
Inspired by the NPC mechanics thread.
So, the set-up is this: the campaigns BBEG (Big Bad Evil Guy) has harrassed your PC group for the past ten levels or so (say, 5th-15th). He's sent minions after you, kidnapped your relatives, killed a few of your group and maybe even mocked you by monologuing from a safe position.
Now at 15th level you've breached his sanctuary and confronted him / her / it. After another brief gloating from the BBEG (the DM enforces a cinematic break, so nobody loses actions / buffs while the BBEG reveals his devilish scheme to return Electrum Pieces into circulation), the initiative is rolled. Tensions are high.
Something unexpected happens. BBEG botches his init. Then it might be your wizard who scores a lucky Disintegrate after another lucky (quickened) Dispel Magic. Or it might be the archer who rolls 20-20-crit and the DM had earlier insisted on using the instakill optional rule from DMG.
Good reason not to use that rule

(the 20-20-20 rule)
Or the Barbarian lucks his way through the Mirror Images, Displacements and Anti-Life Shells and forces a massive damage save.
The point is, the BBEG is stomped on round 1 in a fight that's the pinnacle of your adventuring careers.
Would you feel satisfied?
No. In a vicious game with insta-death effects, he shouldn't confront the PCs on his own. He should have a "cabinet" of NPCs who travel with him.
EDIT: Hmm - to make it clearer, would you feel more satisfied after a drawn out 20-round with your side on single digits, I guess. Which the fight would've been if not for your incredible luck. Sorry for the unclear instructions.
EDIT 2: To make it clear, the BBEG loses on round 1 due to the PCs luck, not because the BBEG goofed up. The BBEG is fully prepared. Buffs up the wazoo, he knows the PCs favorite tactics and weaknesses, as much as the PCs haven't disguised them. His border conditions:
1) He would be around party level +5 -- +8 on the EL scale. If he's less than that, he's not really the BBEG. If he's more than that, he's in some other adventure.
2) He has to stoop down to actually confront the PCs. It happens now because the storyline is exhausted or the campaign is drawing to an end. This has happened due to the PCs determination and actions and has caused considerable hardship to the PCs already (kidnapped relatives, dead PCs mentioned in the text).
Ok... what do you mean by "stoop down"?
The chances of a villain, even higher level, of smashing the PCs on his own is very low. In real-life, group size matters. Therefore the main villain should not face the PCs on his own. Ever. Named henchmen with lots of barbarian levels but without the smarts or leadership ability to lead the villain group might face them on their own, however. (Same with the dragon the BBEG mind-controlled with the artifact that requires evil acts to use.) The barbarian and dragon are not the main villain, even if they happen to be higher level/CR than the main villain.
I'll give an example.
A while back I ran a Three Kingdoms campaign. The main villain was Dong Zhuo, a smart but exceeding evil warlord. He was 9th-level (the PCs ended at 8th-level).
Near the end they faced him, defeated his guards, but couldn't catch him.
On the last day they took down one of his named subgenerals (again, a group battle, again a 9th-level named villain, but he wasn't smart enough to be the leader) and shortly afterwards fought his champion (who was not that smart [Int 8] and fought the group by himself). The champion was 13th-level and had very high Defense. So he lost. That wasn't too surprising.
Designing BBEGs can be a pain. You can't design a BBEG... you actually need to design a BBE
G, where "G" stands for "Group". That's about four times as much work (assuming you're using four villains).