This achievement requires you to kill the enemies on top of a fallen down piece of highway before a bomb is detonated.

You will reach this part at the very end of the Mean Street Echo where there is a small concrete wall with one of the ammo refill points just to the right.

The countdown for the bomb starts as soon as you climb the wall, you should charge your sniper beforehand.

Using charged shots, aim down your sight so you can see enemies marked by a diamond above their heads, fire charged shots towards them and you should take out 3 or 4 enemies at once.

Continue to take out enemies, once you are out of charged shots just use normal ones, or if you prefer, you can climb back over the wall and refill your ammo.

I've uploaded a video of me unlocking the achievement so you can see what to do, its fairly easy. Take a look: