Unfortunately hold time is not cumulative and will need to be done in a single match. To get the flag to spawn, you or someone from the opposite team needs to get one kill. This will cause the flag to spawn in on the enemies body, from which you can then pick it up by holding (X). While holding the flag there is marker above you that can be seen from anywhere on the map, so staying stationary and cloaking will not hide you from the enemy team.
My advice is that you unlock and equip the maneuverability perk which will allow you to sprint longer and faster. With this, just keep moving around the map, trying to avoid the enemies. Having a team that will protect you will also greatly help. If you get into a skirmish, click in the left thumb stick to swing the flag as protection.
Once the timer in the upper right hand corner hits 240 seconds, bleep bloop, the achievement will be yours.
Goodluck and happy hunting!