I don't really see a point in adding in the spell Shell when Proshell does what you want but better. The word "Proshell" also includes the word Shell, which I do believe is just Square Enix's way of combining the two spells without needing to have to actually use Protect, Shell and then (Swiftcast+) Stoneskin II.

What is your reason as to why you would like the spell Shell versus bringing in a White Mage that can use Proshell?