Homefront: The Revolution

The best Stronghold I found to easily get this achievement was the "Hinkin Factory" in the Lombard Red Zone.
First I recommended that you use the crossbow, distractions and RC car bombs.
Once you have completed your preparations, there is a small opening between the fortress and a nearby building where you can use the RC cars to eliminate as many enemies as possible.
Then use one last Hack Car to openning the gates of the Stronghold and carefully proceed with the crossbow.
If the minimap flashes red during a takedown of an enemy, that doesn't void the achievement, but getting the red detection meter filled will void the achievement.
Once you have killed every enemy in the base, deactivate the valve and you should get the achievement.

BUG NOTE: It is possible that more enemies could spawn in the stronghold while you deactivate the valve, screwing you over. It is also possible that enemies can spawn outside the base and somehow see you through walls.


(Video below in Portuguese, but You can see the strategy used)

I found a really simple method for completing this achievement. Which involved no complex routes or RC cars.

Location: Oldtown

Stronghold: Water treatment Centre

Equipment: Crossbow, Explosives

Time Taken: 2 Minutes

Method:

Run around the perimeter of the stronghold until you come to a silver fence with a gap in. Run through the gap, staying close to the walls of the stronghold and right at the back you will find a vent. Use an explosive and blow up the vent but dont go in. Once the vent is broken I return to a gunlocker to save. Then when you return the vent will still be broken and will remain so if you ever need to reload the checkpoint.

Climb through the vent and move into a storage room. There are two doors here a front and side, leave through the side door.You will quickly come to a staircase, there will be one enemy on the ground level. Use your crossbow and get the kill. You will also see two more enemies on the upper walkways. You should be able to kill both from here.

Stay crouched and slowly move up the staircase, once at the top you only need to travel a few feet and interact with the valve. Attempt to limit your kills if the enemy is off in the distance leave him. The quicker you do this and the less enemies you encounter the easier it is.

I tried on nearly every stronghold and as I was playing on hard almost gave up as enemies would quickly overwhelm the base. However by using this method you enter the base right next to the valve, its just a matter of getting up that staircase and taking out enemies.



Hope this helps.

For me the easiest Strike Point to get this on was the Holloway Shopping Center, as there's a stack of boxes on the outside perimeter that let's you get to the top walkway very easily without being seen and does not require you to use hacks. Once you get up to the walkway, there will be a few enemies patrolling back and forth, but they are spaced far enough apart that it's easy enough to take them down without the others noticing you(still I decided to play it safe and use takedowns on them as opposed to using the Crossbow and risk missing a shot and alerting the enemies) and from there you can easily reach the Valve(don't worry about killing any of the other enemies below or on the other walkway, just get rid of the ones on the walkway with the Valve and they'll never notice you when you go to turn it).