The achievement pops at the end of the round as long as you are on the winning team. The games are pretty long, so if you want it fast without risking to loose a few games, be sure to not join a full server so you will be able to switch team if you are on the loosing team.
Switch if the other team really is dominating, i.e. if they control the flag and have twice as much tickets as your team. It is really difficult to change the tide of a game in this mode if one team is clearly better than the other, so you will get the win at the end.
You can switch team in the pause menu (press start) in the Team & Squad management option. You must not be currently on the battlefield (either you're dead or not spawned yet) and the team you're leaving must have more or the same number of players than the opposing team.