Thought about this for a while, and I think there are 2 ways to do this, one invading, one not.
First, the invading one, is pretty easy: go in, redecorate as you like, get out. You might get the result you like or a drooling vegetable and a lawbreaker stunt.
The other one would be more like levelling the playing field. You don't overwrite the victims mind, you just give him enough power (or strip the enemy of enough power) to give him a chance to overcome the invasion by his own free will. Maybe a Ritual involving a soulgaze (or something like it), where the wizard can talk to the victim or give the fear a shape the victim can then attack.