Hmm, one thing that hasn't been mentioned is the... I think it's the 4th law of magic.
Since the laws of magic are about keeping a wizards power over his fellow human beings in check, breaking the rules would logically be somewhat more powerful. I know that doesn't matter with non-humans, but it might be a good way to look at it as a whole.
If you go that route, you are kind of instantly one level above everything, so they don't really concern you as opponents anymore. Now you should be concerned with people and things trying to take you down for what you did and/or are doing.
[This is not meant to start a law debate, I just wanted to point out that in a case like this, the laws should be considered, however you play them out in your game]
How do you model an alien brain? How is it going to benefit the Troll, mechanically? Is it simply an aspect he can tag, or is there something more...like a threshold to bypass?
I think a threshold would be a good way to model it. Like an automatic armor:X or something like it. Or giving him very high skills, if the mental battle takes place in his mind. It should feel a bit like doing brain surgery on a nuclear rocket, on a boat, in a storm, with your eyes tied behind your back, while trying to smell the color 9. Most times, I would not even let you do it, you don't even have an idea of where to apply your mental force. Even on a tiny and insignificant fae like Toot.
Characters like Molly might be an exception, since she is not only very good at that, but she had training from a shide as well.