Warning: Spoiler Alert
This achievement IS unlocked through the main story. You get this for beating the Arkham Knight for the second time, down in the tunnels. This is relatively straight forward and is essentially Pacman on a larger scale. When you first see the Arkham Knight in the tunnels he will be in a huge tunneling vehicle, don't shoot him when you gain control because it won't do anything, instead go into guard mode (LT) and quickly spin the vehicle (in a 180) then speed down the tunnel. Shortly thereafter you will get a clip where the Arkham Knight runs into some mines on the side of the wall and the tunnel collapses on him. Now you are stuck in a tunnel system, which really isn't all that complicated.
You will see on the map 3 orange parts, these are the parts with the mines that you need to coax the Arkham Knight into. To lure him simply sit the batmobile in the tunnel entrance (of the mines you intend to lure him into) and shoot at him to gain attention. He is VERY VERY quick, so don't even wait for him to react, shoot, spin and drive like crazy (he does slow a little the closer he gets). Each tunnel will have a few obstacles to dodge on the way to the mines, once you pass the mines, as before, he will trigger them and the cave will collapse on him. Each time this happens (including the first time when you first see him), he loses 25% health. Once you have got him the fourth time you will get the achievement.
Also don't worry about him killing you too much because there is a checkpoint between each time a cave collapses on him.
This is a really easy achievement, although I gotta say it is pretty stressful when he is on your ass and you know one mistake means your death.