Best Spells to Defend Against Casters?

[NecessaryWeevil]

In NWN2, Spell Mantle was a good way to defend against casters. 3.5 Doesn't seem to have similarly-universal defensive spells once you get beyond Globe of Invulnerability. It seems like once your opponents are using 6th level or higher spells, you're limited to stealth, counterspelling or buffing your saves as defenses. Am I wrong? What sort of defensive spells would you take if you expected to be facing CR15 or above foes but didn't know specifics?

(I'm a 15th-level Malconvoker, so I learn spells as a 13th level wizard but cast as 15th due to Practiced Spellcaster. However, feel free to mention higher-level spells so I can aspire to them too).

Thank you!

[Lightlawbliss]

In optimized dnd, immunity is always the end goal. Mindblank is a god in the immunity department, as is any method of becoming and outsider, construct, or undead (or better, but methods of becoming those get sketchy)

[Flickerdart]

The best way to shut down your opponent's casting is to nip it in the bud and become a sweet counterspelling guy. There are loads of feats and stuff to counterspell without wasting your action on readying, but if all you have are spells, battlemagic perception isn't a bad one to have up (it can be expended to counter as a free action which is all kinds of neat).

There are also Spellblades which you can key to specific spells, but they're items and not spells themselves.

[Grek]

The best defense is a good offense. The best offense is overwhelming, irresistible and surprising. Therefore: prepare Nerveskitter, Dimensional Anchor and Force Cage. The second best defense isn't to be where the fighting is. Therefore, prepare [Calling] spells. The third best defense is to leave if attacked. Therefore, create a Plane Shift Contingency.

[Zaq]

Lots of casters rely on ranged touch attacks for their nastiest spells. Friendly Fire (Exemplars of Evil) is an absurdly good way of shutting that down. Even True Strike won't get you past Friendly Fire.