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. 
Well, part of the problem with going around behind them is their Teammates (if they’re actually competent). That, or not even knowing where the hell the sniper is.
Well, if the two teams are uneven in terms of skill level, then that’s something that can only be fixed by switching around the team members…