If you have trouble killing a sniper, go and snipe them. They’re too focused on what’s in front of them, so they can’t tell if someone goes around behind them with a shotgun, or is out of their field of view and snipes them. Don’t go ranting post-game that you couldn’t kill this one guy and that he was murdering a team like no tomorrow.
Also, killing is an integral part of Lost Planet. If you don’t like being killed, don’t be on Lost Planet. Easy.
My two cents.