This achievement is much easier since TU3, just play the Guardian playlist, and when the game starts rush the enemy leader and kill him to become the leader if you are not the leader to start with, Once you are the leader plant grenades preferably frags but if not smoke and get one or two teammates to stay back and protect you while the other members of your team go and kill the enemy leader.
This achievement is also achievable in Social, which is easier than Ranked matches as you will sometimes get in a match with just yourself and 9 Bot players. (Credit to UlteriorDesert for conformation of this)