I can understand your pain. The Thorn is a very infamous gun because of its ease to use and its perhaps unreasonable power. I suppose it's a matter of knowing your opponent (in this case that is Thorn, not its wielder). The Thorn doesn't have ridiculous bullet velocity, so jump a lot. And remember that there's no shame in running away. Also, its accuracy isn't too good when the user is in the air, so if he is, understand you have a recognizable advantage. And remember that the Thorn is particularly dangerous with headshots, so do anything you can do to close the distance so that the Thorn's remarkable range doesn't get you, and so that the user has to rely on hip fire, i.e. not headshots.