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I encourage you to check out the Way of the Wicked AP by Fire Mountain Games, it goes all the way to 20th lvl. I would say it does a good job at offering advice at dealing with high-level spellcasters.
I second this.
WoTW is written for evil characters, but the principles of dealing with high level casters remain the same.
Essentially the latter part of the AP is very much a sandbox that embraces the fact the high level characters have extraordinary abilities to 'bypass' obstacles.
There are no linear dungeons, and encounters are designed so that the PCs would either want to face them, or have no choice but to face them or have their resources threatened.
On the whole the entire AP is excellent from start to finish.