Thorn can easily be countered by any gun in the game, especially vex, last word and red death. If you use red death, you can kind of stop the dot and kind of get your health back. If thorn two shot head shots you but you kill him as well, the dot won't kill you. You will get part of your health back. Heck, I even kill thorn users with AR's sometimes. If you move around and try to get closer to them they will miss a crap ton of shots.