Easy as it sounds, just capture 10 flags in the game mode frontlines. I did this during early access,
so there were no servers for frontlines at all (only conquest) so you have to play the infantry based playlist (TDM, Domination, and Frontlines). I will update the guide when they add frontlines servers..
EDIT (Thanks to Mr Dan Jones):
Just a tip if you press X when highlighting the Infantry Playlist you can see all the servers and what gamemode is currently active. Makes it a bit easier to find a Frontlines game.
Anyways, depending on the map you will have a set number of flags that you will have to capture and push your "frontline" towards the enemy base. This will take roughly 2 matches to complete (I got 8 flags on the map Twisted Steel because there are 5 total flags). Even if you or the enemy capture all the flags, the game is not over because you have to plant explosives at the enemy base, which is very difficult to do due to the spawn points. when you fail to plant the explosives (or vice versa) the last remaining flag opens up again so you can keep capturing until the game ends.
Please note, you only need to be in the flag area when the bar completely fills up and your team captures the flag, so try to survive when you are in the zone.
All in all, very simple achievement. let me know if I need to clarify or improve anything, thanks.