This achievement requires you to do two separate tasks which span across two levels.
Load up the level DEFENSE
in chapter four and stand your ground with the "Children of the Underground" squad at the start. After this event you will walk past the commander who is wounded on the floor. Make sure to stay close to him until he finishes talking as walking past him makes you unable to progress with this achievement.
Now he will tell your to broadcast a prepared message atop of a radio tower in case the station would be overrun. Make sure to take the cassette tape he gives you because without it you won't be able to broadcast it.
Now play all the way trough the level and continue on to the next level, which is called OUTPOST
. Keep running/sneaking across the level until you have gone all the way up the staircases. After the stairs you will head right to progress and you will spot a Nazi enemy running from a Demon(flying monster). Next to this scene is a large building with numerous enemies which you have to climb all the way to the top where you will find the radio tower.
Now there is an easy shortcut. From where you approach the building, there will be a hole in the wall right before it. Go trough this hole and you will spot an excavator which you can vault. Slowly make your way across it and you will spot a ladder which goes all the way to the top of the building. At the top make sure to make a dash to the little hut you will spot on the right side and take cover in it because there will be another Demon waiting for you and you don't want to play games with it.
Inside the hut will be a radio which you can press action on it. This will load in the cassette and play the pre recorded message, unlocking the achievement.
Here is a video to illustrate:
While on top of the building you can also go for this achievement:
Trow a spiked/sticky grenade at the demon and make sure it hits it. It will stick and blow up, killing the demon. Now reload checkpoint and repeat to unlock the achievement as reloading checkpoint does not reset your progress.