I don't like this quest. Anet has clearly tried to make it hard to "cheat". If you clear all the mobs before talking to Rahlon, you will hear the quest updated noise. Then when you actually take the quest, you will find that the raider is a long way ahead of you, as if you'd been chasing him. He does wait though, there are checkpoints that he waits at, so as people have said you can avoid these and approach from the other side.
I did it how I presume it was intended, with running skills and Crippling Shot. I had to get Acolyte Jin to do it, which was no problem. But even with Dodge and Crippling Shots, I still only caught up to him when he was almost at his destination, and only Jin and myself were still alive, because of all the enemies on the way. The fact that they never stop chasing doesn't help. Nor does Imagined Burden.

Oh and the fact that Rahlon nearly always dies just before you kill the raider.
I think it is my choice for worst quest in the game.