(Bryn talks a big game but deep down he's scared of being powerless. I'd say Sebastian exerting his power over Bryn and forcing him to do something would work. Or, like, forcing him to submit to something. Also, Bryn's kind of a wimp when it comes to pain.)
Bryn was humming quietly to himself as he picked up a shard and wedged it into the trailer door, grinning when he forced it open a crack. Even in this powerless state, he was a good deal stronger than any human. Inch by inch he pried the door open until he could stick his hands out the back and wave them around.