Counter? Take cover and aim for the head. With a weapon like mida, it's a lot easier for you to aim steady headshots than him. And for crying out loud, if you're damaged, don't try to go all Matt Murdock and take him down while you're wounded. That's a dumb idea in any situation. If you're damaged, regen. And then fight. I'm not a spectacular pvp player by any means. But I've killed many players who had thorn. It really isn't a big thing. Don't rush control points, don't stand there and have a dps race, move around and find the weak points. Sometimes that thorn user is just a noob who thinks that his gun makes him invincible. Out play him.