The hardest part about this 'chieve is bullet drop, it never really tells you compensate for it nor really exactly how to, so this is how...
Aim 2 notches down for the first group of rpg's in the wide building on the left.
Wait for the helicopter, Don't shoot the enemies w/ the ak47's. When the helicopter gets there, you should already be looking at the tall tower w/ the final 2 rpg's at the top..
For the 1st enemy on the right, aim 3 or 4 notches down. Don't even shoot the 2nd enemy, let the helicopter take him out, you will still get the 'chieve even if you don't kill the 2nd enemy on the very top. If you do want to shoot him, aim about 5 notches down.
Edit: Did I get the notches # wrong? Or what exactly isn't right about this solution? Please leave a comment and I'll update it.