Destroy 3 tanks with a single 105mm shot in "Iron Lady."
As soon as you take control of the AC-130 at the beginning of the "Iron Lady" mission, switch to the FLIR (infrared visor) with the . You should be set automatically set to the 105mm, but if not, press to swap between the 3 ammo options. Now if you look in the center of the circular area (use the to zoom in and out), you'll spot 2 tanks side by side. Then look to the left of these, and you'll spot a 3rd tank moving up to them.
All you need to do here is wait for that 3rd tank to be along side the other 2 and then simply take a shot right in the middle of them all. With them being all so close to each other, the explosion of everything will take all 3 out in one go and unlock your achievement.
There should be a checkpoint EXACTLY where this achievement can done. Only spot I saw where 3 tanks were close enough to destroy with a single shot. So if you miss, got to last checkpoint and keep going until you get lucky! BTW, the AC-130 is a fricken beast!