This is easy if you play with friends and agree how to do it. All flags but one left must be captured. While the last flag is slowly being captured by the enemy team, rush in next to the flag and you'll block the capture, getting this achievement. This can also be done normally while playing against other players but requires a mix of both luck and preparedness from your side. Keep an eye on which flags are taken already by the enemy, then stay near the last flag they need. Once they are trying to capture it, rush in to block.