| Jaelithe |
Great villains have great flaws ... and even brilliant people forget to tie their shoes, at time. It's asinine to assume that an enemy who doesn't perfect every aspect of his plan is going easy on the characters.
If he or she crushes the party, players are pissed that their defeat occurred with ease, and yelp, "That's not FAIR!" If they defeat him, then players are pissed that he wasn't sufficient challenge or that the DM let them win. (Obviously I'm being extremist here.)
My solution: An assortment of opposition. Some are mooks. Some are clever but not wholly committed to your death. (It's just a job.) Some are your sworn enemies, but not too bright. Some are brilliant, ambitious, powerful and see you as a stepping stone to their greatness. Some at first despise you, then grow to respect and even admire you. (Yes, it can happen that a villain wishes to prove him or herself superior, but has no desire to kill you. Perhaps you spared her once, and she possesses honor. Perhaps he has a secret crush on you. Perhaps he enjoys your conversations on the nature of good and evil, and would miss them if you died.)
If every villain were subtle, mighty and determined to dance on your graves, well ... that'd be boring, "unrealistic," and predictable.