In other words; there is none.
I was reading another thread and the thing that stuck out most, to me, was the lack of defenses against magic. Seriously. We have evasion and...moving. Spell resistance only goes so far, especially in end-game, and that does nothing against the vast majority of damaging spells, which, at the end of the game, can be devastating.
When, in any D&D game, novel, or anything, has any wizard or sorcerer not gone without their spell mantle or actual shielding spells? It's hardly dignified for an archmage to dodge behind a rock, anyway. Defensive spells like nightshield and shield are near useless at the end of the game (except for absorbing magic missiles, which aren't widely used) because their resistance bonuses don't stack with anything. Protection against Energy is about as useful as shielding yourself from a nuclear attack by standing behind a wall; it'll work for a moment until it melts. And I'm not talking about blur, displacement, or stoneskin; those are defenses against physical attacks, not magical.
I say we need to add lesser/greater spell mantles to the arcane spell list. Not powerful ones, just enough to shrug off a few spells so that arcanes don't get obliterated when they shouldn't have to. You could have it act like stoneskin, but stronger, and absorbs magic damage of any kind; to balance it out you could give it a lengthy cooldown to prevent complete immunity to spellcasters.
And no, losing casting ability to take two levels of rogue or monk is not a solution, that's a workaround.