No need for YouTube! The game sets up 3 kills for you. Right after you take out 4 guys at a truck your friends open a big sliding door. Two enemies are the other side, take one out before your allies kill them. Next you go upstairs and split into pairs. Run ahead and shoot a guard on the ground floor before the are all taken out by your team. This will be ur 5th kill and the Cheevo will pop. You need to get these as you go into the black site straight after. If you miss the kills reload checkpoint, do NOT go towards the big pressure door as it will give you a new checkpoint and you'll need to restart the level.
This achievement refuses to pop for me. I've killed the guy you jump down on with a knife, the guy at the trains, one of the guys at the truck, and the 2 guys on the other side of the sliding door as it opens, and I've repeated this multiple times, no achievement. I've also ran ahead into the next segment like #4 above suggested after killing those 5 and killed an additional 3 unaware enemies, still no achievement. I'm about ready to give up on this one.