When I decided to go for this achievement I was surprised to the lack of solutions considering the high TA ratio (2.74 at time of writting this) and difficulty involved. So I am writting this for anyone who is looking for tips and advice.
Technically speaking this should not be difficult considering in this game you do not encounter any boses at all except for endless hordes of bots with guns coming at you continuously. However, while in easy difficulty it takes a lot of bullets to drop and eventually kill you, in Extreme it takes three! First makes the screen gray, second makes it red and third puts you in the grave.
I will be the first to admit that I suck at FPS games and I am not a constant headshot guy. If you are, like some of my friends, a person who did Mile High Club of MW1 in the second try (took me 200), then do not waste your time, everything I list here you already know by heart and your are probably laughing at me for stating the obvious.
So here goes nothing:
To Co-Op or NOT to Co-Op:
This is probably the most important decision you will have to make! If you play it co-op let me tell/warn you that like Halcyon Pure says only the host will pop the achievement. However the guest player will also unlock a certain number of chapters on extreme as well. So what you will have to do is go back to the menu and look which chapters are unlocked on extreme and which not. Then switch roles and you host only those chapters in any sequence to pop it.
<< gamer JollyBunny24 reports that it popped at the same time for him thus no need to repeat anything>>
In my personal opinion that is the wrong choise though. Even though you might have friends without real life-commitments and are willing to help you for as long as needed, that also means you will have to play certain levels twice resulting in double the effort. If you decide to do Extreme on your own you will be happily surprised to find out that your bot partner is "RAMBO". What I am trying to say is that your bot Kane (or later Lynch) WILL NEVER DIE and WILL NEVER RUN OUT OF BULLETS. Yes, you read it right, they might shoot him and drop him but he will get up and continue killing for you. The only trade-off is that if you abuse that, by not doing anything he will either come next to you, staring you, or even worse let some bots "slip by" him to come kill you. Thus forcing you to shoot them down. With that in mind, cover and gameplay are our next important section.
Finally if you go alone on the Extreme I suggest you do what I did. Start a New Game on Extreme so that whenever you quit you can just Resume Game to the exact same spot you left the game. Loading another checkpoint or chapter might require you to redo a level or even worse might alter the actual difficulty you are playing at.
Gameplay - Cover System:
The cover system is what makes this game so frustrating. If you assume a vertical cover position you will die in seconds as if you were not in cover at all. The solution is to do what CANNON FODDER52 is illustrating on his video and you are sorted. I have to add at this point though, that horizontal cover DOES work. So if you see a bench or barrier and drop into cover horizontaly you are safe no matter what the enemies are packing.
The annoying part is that your enemies will use that cover system extensively as well, thus forcing you to spent minutes waiting for them to pop their head up so that you can shoot them down. If you see your bot shooting a wall that means there is an enemy hiding behind there. If the enemies refuse to pop their head up, move your LS left and right, as if dancing, and they will immediately pop up to shoot you.
The good news is that every section has a certain, finite, number of enemies. Meaning if you take it slow its inevitable that you will walk freely to the next area. How do you know that you killed them all? A little "Saving..." will pop in the top left side of your screen. Some times you will hear your bot screaming, "Lets gooooo", DO NOT. I promiss you there is one or two bots left waiting for you to make a move and have a bullet with your name on it. Wait for the "Saving..." and then make a move, otherwise you take all responsibility.
Most weapons are quite accurate so its good news. The only ones that suck are a few submachine-guns that have a round target/aim lock. Avoid them at all costs. Assault rifles rule! Short bursts ONLY, no need to be Rambo. The funny part is by just shooting at the enemies you will get a lot of lucky headshots. I was getting headshots with shotguns from a range of 30 meters. If your bullets actually hit an enemy you will see an "x" on them for every bullet that hits. Also always press the down so that you can locate the explosive barels, since they are always strategically placed around large groups of enemies. Shoot them and enjoy the firework display. Finally, as a great guy wisely said (CardoP), make sure you always have looooots of bullets.
There are 11 chapters and I was only stuck on a couple of them. I died lots yes but I never gave up. The further you move up you will notice that enemies take more bullets to die. If you decide to "Storm the front" 7 out of 10 times you will be happily surprised. Running when you are not supposed to you will many times catch the enemies off guard and they will just stare at you, not doing nothing.
For example, I was stuck at the start of LEVEL 10 "Resurrection" when you are supposed to sneak behind a sniper, grab him, kill him and then kill the guards. I tried going ultra slow "ninja" style but he would see me 5/10 times or then the 15 bullets on his weapon would not be enough to kill the enemies. When I had enough of dying I run all the way to him, grabbed him, killed him and then run straight to the door, lifting it and passing to the next section with the bots not even seeing me !!!
In LEVEL 2, you have to keep glazer alive but if you reach a savepoint and die you will see that his life get restored.
I died a lot in the start of LEVEL 7 "The Deal" because you start with 20 or so enemies and wooden boxes that get destroyed fast and thus cannot be used for cover. After many trials and errors I found that if you go immediately running far right you will find a solid forklift to take cover behind.
Finally, in LEVEL 11 "One Way Ticket" in the very last section you have to actually run to make it to the plane before it takes off. I found that shooting a shotgun from the hip will give you the lucky headshots you need to make it in time and even provide you with a couple of spare seconds so that you can stop and shoot the last two annoying dogs!
I hope I helped a few people to get this achievement like me, and please do not leave negative feedback out of spite,tell me whats wrong and I will edit this solution.
Thanks for reading :)