Even still now it's a notorious achievement, I'm so glad I got it last week!
Just keep trying, it's really a mixture of luck and skill, knowing where the enemies come from and where you best throw a flash bang is important, but having the luck that one last bullet that would have send you back didnt get ya is needed as well
Don't blame yourself when it's not working on your first attempts, just keep trying and eventually you'll get it!
Ps. For my attempt I always tried to pick p a p90 if I found one along the way, the larger mags and faster reload helped out A LOT! But don't waste your time by looking for one either, again, a bit of luck helps...