A few clarifications: Brand is an ability acquired during regular story progression. It is indeed only combat branding that must be bought and, hence, requires Level.
Power Vacuum in Middle-earth: Shadow of Mordor (Xbox One) is not Time Sensitive. While the original comment is correct, you only have a limited time to get the achievement after you have completed the set-up required, there is no predetermined time frame. Let me explain a bit.
If you kill a captain another Orc or Uruk will try to advance his position and take the now vacant spot. This takes some time. However, how much depends on what you do in-game. If you happen to die, time will advance almost instantly and most positions will be filled without you having a chance to get the achievement. But even if you don't die, you could have to travel to far so that some position might already have been filled again. In essence this achievement boils down to not slacking or dying once you completed the set-up.
Therefore, as there is no specific time limit, I petition to remove the flag.