Psion said:My Players' fighters not being able to kill things is not the problem. I almost have to make sure there are creatures in the game with considerable DR just to ensure the spellcasters get their licks in.
I think increasing AC's of enemies helps quite a bit. My party of 13th level characters just recently fought a Gelugon (CR 13). With Unholy Aura, the creature's AC was 36, which was more than enough to keep the fighters from having an easy time of it. In addition, they couldn't effectively Power Attack, so the DR made a big impact. If I had wanted to make them nearly ineffective, all I would need to do is add some Bracers of Armor +5 (or so). There ya go, CR 13 and AC 41 - "Swords are no more use here".
Most of the demons are shown unequipped and are very, very tough for their CR. If you add a few defensive items, they begin to fall into the realm you're looking for.