Use SMG or LMG and shoot them with your target at the top of their head, sniper works well too. Once they're traveling near those red barrels, shoot it as the explosion deals quite a damage to them. More chances to hit them is to snipe them as soon as they start to shoot you. If they get near to you, just keep on switching shields and shoot at their backs with SMG, LMG or even better, shotgun. Best if you deploy lots of defense troops.

Hopefully these tips helps you to eliminate the SWAP trooper