Coming from a Thorn user, it's pretty simple. With Thorns, you don't want to draw out the engagement. If you give proper time for the burn from one shot to the next, you maximize its damage potential. You want quick bursts of damage. Know when to fold 'em. Always attack their blindside. Thorn is incredibly strong in close quarters. Not Shotgun strong, but a Thorn shot and a melee is usually enough to kill. Attacking their blindside works on everyone, and so does burst damage, but it's good tactics nonetheless. Avoid attacking from low cover, things you crouch behind. It's better to attack from behind things like a wall or whathaveyou. Your cover might not be perfect, due to your head peeping just enough for them to finish you off after that first shot you're recovering from. Headshots are key to Thorn users. Try to limit us in their deliverance. If available, attack in teams. Again, normal Crucible tactic, but the Thorn's Time To Kill is lower than other prime weapons, but its "time to hit that magical threshold where you'll probably die" is pretty low. If the Thorn user has to focus on one target, he'll probably die. The Thorn user has to, in order to actually get a kill. Learn to sacrifice in Crucible.