Garviel might well be a significant challenge. He's over 120 years old, and as a Space marine has certain defenses against over psychic control. He can be mindread but due to 1: Garviel's notable canonical resistance to mind-controlling effects and general purity, and 2: his being a space marine who all have a bit of resistance in this area, he will be bitchhard to physically control. (Though not impossible.)