Elder-Basilisk
First Post
Nope, not every bad guy will have True Seeing. However, a lot of major bad guys will - major demons & devils, powerful spellcasters, etc.
Curiously, major demons have true seeing, in 3.5 though, most major devils do not have it. Greater mirror image is not a defense against a marilith but it works just fine against a cornugon.
Also, while we are discussing ways characters can boost their ACs, don't forget the recitation spell. +2 or +3 luck bonus to AC is a very handy stacking bonus. It can also be quickened productively around 17th level or so. A quickened recitation is a reasonable way to spend an 8th level slot. (Or your 7th level domain slot if you have the purification domain).