I found a more consistent way to pull of that first turn, instead of using a lancer to break the sandbags run a shock trooper up to them and throw a grenade, from there run a scout up to the same spot and fire on the enemy shock trooper getting the team attack. this allows you to use your sniper to clear the enemy sniper on the rooftop rather than being reliant on the like 10% chance that your scout will be able to pull off the kill.
