just run away and LoS them
1) Find a pole, or corner, or anything that blocks LOS
2) Stand near it
3) Use instant cast ranged attacks on sniper
4) When you see his cast bar start to move, step behind cover
5) His casting bar says "Failed" because you LOS
6) Step out and attack again
7) Go to step 4
You will test their patience since they wont be able to hit you with anything that's channeled.
Enjoy eating an Orbital Strike while behind that pole
There are ways around LoS, a good sniper/gs will get you out of it, forcing you to eat the burst or completely disengage and go somewhere else. Win-Win situation for us.