Touc
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Max out Spellcraft
Improved Counterspell
Parry Spell
The Advanced Player's Guide has rules for a subset of the Abjuration School called "Counterspell." The ability is iffy (because it's touch), but you can, by melee touch, put a disruptive field around the caster. The 6th level feature gives you Improved Counterspell and ability to use it as an immediate action once a day.
While others may say it's better to go on offense than defense, your caster can shine when it comes time to take down a caster and do whatever it is casters do when not facing other spell slingers.