This is story related and can not be missed.
*** Spoiler - click to reveal ***Towards the end of the game in Old town. You are instructed to activate the amplifier at the top of the tower. Just climb to the top of the tower after you get by all the zombies. At the top platform of the tower; go to the Amplifier and press X; the achievement will pop once the cut seen completes.
If you climb to the very top above the Amplifier there is a collectable flag.