You can't kill them, and they can't kill you with one person either.

I just refresh my totems, or let a few random attacks proc water shield, and wait for them to get bored. You waste more mana/cooldowns trying to hit them when they're just going to run away. Shoot to kill or don't shoot at all.

If you resolve yourself to be the impossibly patient opponent, they'll eventually get frustrated and a) make a mistake -- jump on them with burst damage and take one out, or b) decide that their tactic "isn't working" and do something more stupid.

Personally? If I know someone is going to just LoS/Pillarhump, just purge spam them or throw a bunch of cheap flame shocks and watch their healers rush to overheal them. They waste mana rebuffing, while you get to laugh at their paranoia.

Of course, if they're full s4 they could just be playing with you. I haven't seen anyone with anything more than s4 helm.