Three choices: 1. Hope the Sniper isn't good enough to hit you while you desperately hit every single key on your keyboard to move as awkwardly as possible. 2. Hide behind the same wall during the whole match or take comically long flank routes so you can kill them in the case no teammates are around him. 3. Start maining Sniper as well since he's literally his own best counter to kill the enemy ones and make the game more painful for the rest of the opposite team while you're at it.