It seems extremely narrow for a -3 power. While it is very strong in the situations where it is useful, the fact that it will come up fairly rarely would make me say this should not cost more than 2. Also, any magic user who figures out this power will just not use magical defenses anymore, and while this is pretty much equivalent to bypassing them, it does make the last part weaker. Why does it cost a fate point each time you activate it? It is certainly no more powerful than ACaEBG which (as far as I can tell) costs 3, and it is a hell of a lot more narrow.
Also, you said that it only works on spells "(costing mental stress)" does this mean it does not work for enchanted item's that provide magical defenses?