Hey Guys, This is My video that I made to help you and in this guide i will quickly explain what you have to do to unlock the 'No Man Left Behind' Achievement. Very simple but can be missed so on Act4, Chapter 3 as you are running through the collapsing building and you see the timer start to count down then you have to haul ass, you will encounter to enemies which you have to kill right next to the elevator thats it simple really wasn't it.