Give her the wrong kind of Algollen shell. The local Runes (the fuel of magic) don't like the kind of magic that Ravenna practices, so she has a hard time using them. Instead, she relies on Algollen shells, which can hold Runes inside of them. There are many different kinds of Runes around the world, so she simply imports Algollen shells grown near Runes favorable to her magic and uses those.

Tell her you've got a useful Algollen for her and then watch her reaction as she opens it, absorbs the Runes, and her magic tanks in effectiveness for a while. You also might want to duck. Or not, now that her magic is weaker. :P