After you capture any flag, wait at the base, and as enemies come, kill them. Hide behind a wall and crouch to be covered. If you also wanted to also get the "Attacker" achievement, you can capture five flags, then wait and kill any enemies that try to attack the base. A good strategy is to go to a base with a yellow X over it because this base is a point of interest, which means many people will be going for it. Take a tank or other vehicle for extra safety and either capture the flag and defend or defend one of your already captured bases. If the +3 defend bonus when you kill the enemy does not appear, it will not count as a defend.