how about a three pip spell that you cast on yourself like a cleanse but instead of breaking the weakness it converts it into a shield...
so a 25% weakness would be changed to a 25% sheild and so on...
It's not a bad idea.
My suggestion would be to make it 2 pips, as 3 pips is too costly (2 power pips) for a spell that's that situational.
This would counter Efreet and Lore spamming, so I'd like for it to not be a Fire or Balance school spell.
It's defensive so it should probably be an Ice school spell. However Ice in pvp is already very powerful, so I can't really see that going too well with the community.
It can be thought of as offensive though. Getting rid of a weakness (-25%) is the same as gaining a 33% blade (0.75^-1). So it could also be a Storm spell, something that would be very well appreciated since it lacks a lot of utility.