Now that the cap is 20 regular stoneskin just does not cut it, Greater stoneskin and/or premonition would be a great benefit to arcane casters.
Premonition
Spell Information
Spell level : Innate level: 8, Druid: 8, Sorcerer/Wizard: 8, Other: Cleric with Time domain 7
School : Divination
Components : Verbal and Somatic
Range : Personal
Target/Area : Caster
Duration : 1 Hour * cLevel
600 Rounds (cLevel).
Save : Harmless
Spell resistance : No
Premonition allows you to see a few moments into the future. This grants you damage reduction 30/adamantine; the spell absorbs 10 points of melee damage per caster level before collapsing.
Greater stoneskin
Spell Information
Spell level : Innate level: 6, Druid: 6, Sorcerer/Wizard: 6
School : Transmutation
Components : Verbal and Somatic
Range : Personal
Target/Area : Caster
Duration : 1 Hour * cLevel
600 Rounds (cLevel).
Save : Harmless
You gain damage reduction 20/adamantine. The spell absorbs 10 points of damage per caster level, to a maximum of 150, before fading.
These spells would help increase the survivability of arcane casters end game and would give us something a little more unique now that every other person has a stoneskin clicky or can UMD a wand.