As far as I know, this method (shown in my video below) is the simplest way to get this achievement. It certainly is one of the earliest places in the game it can be acquired so easily. Mission is 'Blackout' and difficulty is your choice.
Sometimes Gaz and Price move forward and kill the last 2 enemies before you can. To prevent this happening I have found that walking to the right and onto the small rise prevents them from moving forward or firing. The main thing is to be swift, keep your distance and don't alert the guards. That seems to be what sparks off Gaz & Price.
Either way, it's no trouble to hit start and restart if you need to try again.
The video I made should help.
Edit: video re-created in glorious HD!