Otherwise known as Back Stabbers, these folks can be difficult to deal with because they are friendly and agreeable to your face, and will always deny any hostile intent, yet they find ways to attack you invisibly, or when you’re not looking. The best way to deal with this passive-aggressive behavior is to surface the attack: that is, let the sniper know that you realize what’s going on, and if you can, point out the sniping behavior as it’s happening. This will likely make the sniper mad, but usually the attacks will diminish over time if you confront them calmly and consistently.This is instructive for all you out there who may be dealing with a sniper or a bully. Or a narcissist who is a sniper and a bully. Don't take it lying down. Even though some of the bystanders will scream at you for daring to assert yourself...keep the exposure front and center. Take it to the little sweet-faced backstabber. Don't pretend you don't see what is happening. Don't think that by keeping your mouth shut that you are somehow rising above it all. There is no virtue in that. Bullies smell red meat if you do not stand up to them. The more you take their crap, the more crap you're going to get. Bullies are cowards. Period. As long as they feel they have support of others they can act all brave. But if you are fearless in exposing them they will seek an easier target. You won't be worth the pain you're causing them. The sniper is a particularly cowardly version of a bully, therefore your efforts to expose them will be particularly effective.
So, armed with that knowledge, go make a scene. Calmly.