They need to either reduce the burn amount of dot or have it cancel if you kill the thorn user. for you can easily spam it to the body for 3 shots for a easy kill or just hit them twice in the body and forget them and let the burn damage do enough that when someone else kills them you get an easy assist every time. I use the thorn when ever i do the 50 assist bounty and it is cheap doing so but so easy to do.