The Drop Wall ability is unlocked during natural progression in the campaign. Play the story normally, and you will eventually unlock this achievement. For more details, see below.
*** Spoiler - click to reveal ***I'm actually somewhat foggy on exactly when I unlocked this, but I believe my current assessment is correct. During the mission at the Banished Excavation Site, after you've destroyed the drill, you should find a deceased spartan. The Weapon will take a look at them, give you some brief information about them and how they were killed, and then give you access to the Drop Wall, an ability they had been using. If anyone can confirm that this is the mission you unlock the Drop Wall in, I'd greatly appreciate it!
Happy hunting!