Stoneskin: DR 10/+5 up to 10 per CL max 100 (unchanged)
Greater Stoneskin: DR 20/+7 up to 10 per CL max 200
Premonition: DR 30/+9 up to 10 per CL
Endure Elements: Resist 5 to a max of 5 per CL max 50
Resist Elements: Resist 10 to a max of 5 per CL max 75
Protection from Elements: Resist 15 to a max of 5 per CL max 100
Since so much can bypass with +5 or better weapons from about the Oasis onwards, and make the elemental resist spells actually useful by scaling.
Alternatively, just make the stoneskin line operate in the same way as the elements so they give resistance instead of reduction, then tinker with the amounts to balance.