Army of Two (EU)

As opposed to what the above solution is stating, you get this achievement not for going into overkill 75 times, but for killing 75 people with overkill on.

Have your AI partner on aggressive, or co-op partner using blind fire, with an aggro heavy weapon to draw up aggro. It helps if you do not let off a shot. When your character goes transparent, it might be helpful to sneak in closer to the enemy - especially if AI is on, in which case the AI will automatically try and kill some enemies with overkill on.

When the first aggro bar fills, you become invisible. When the aggro bar fills again (the red bar within the bar) you can go into overkill mode. In overkill mode, you are completely invisible and do twice the damage. Get in close to the enemies and kill as many as you can. Kill 75 this way to unlock the achievement.

Be prepared; In overkill the AI automatically attempts to kill as many enemies as possible, and can often take out more targets then you would like, stealing the kills from you if you are not already close enough to finish them off first.

Startup 'Aircraft Carrier' on Recruit with either AI or a co-op partner. After landing on the rear of the ship and completing the back-to-back sequence, set the AI to 'Aggressive Advance'. Due to the large number of enemies, the AI will run forwards to fire at the enemies. After his first kill place him in 'Aggressive Stop' so he can continue firing but not get any more kills. Once the two bars have filled up, prep yourself in a position ready to get multiple kills, and finally, activate 'Overkill'. You can do this several times during the beginning of the mission if you time it right, then reload your checkpoint and you should have it in no time.

If you choose to do this achievement co-op rather than with the AI, make sure that the person going for the achievement is hosting. My boyfriend and I wasted a lot of time setting up his kills (with me as host), only to find that none of them were counting. We are sure of this because we were counting his kills. After switching to him as host and retrying he was able to get the achievement.

We did this on Aircraft Carrier on recruit, right after the checkpoint after the back to back. We can confirm that it is ok to simply restart checkpoint rather than having to die first and then start from the last checkpoint. The game will still count your kills.

To go into aggro overkill and thus put him into stealth overkill I would kill one enemy and then hold the RT down as it fills up your aggro bar quicker to keep firing, even if you're not hitting anything. Then once it started to fill up with the deep red colour I killed one more enemy and this would put us into overkill. Hit the A button and you're on your way.
Once I went into aggro overkill I found it best if I kept firing to keep my aggro up but just fired away nothing to give him free reign of the enemies. He also found it much easier to simply use the Grenade
Launcher (MLK I think?) for much easier kills and he found he was able to get 3, 4 or even 5 each time.
Then he just hit restart checkpoint and we repeated this until he got it.

This solution is based around playing coop.

Step 1: Play through mission 3 Iraq until you get the checkpoint on the bridge while you pursue Ali Youseff.

Step 2: After crossing the bridge Rios and Salem talk about coop sniping the 2 enemies by the barrels.

Step 3: Kill them, one person should then hop on the turret. You cannot get overkill while on the turret so fill up your aggro using it then get off.

Step 4: Start shooting with your primary gun and you should quickly have overkill.

Step 5: Activate overkill and Kill the enemies in the surrounding area and reload your checkpoint( Remember to take it in turns unless only one of you needs the achievement). The checkpoint is just behind the turret so the kills are much quicker than other solutions.

Step 6: Do this until you have the achievements for each player then swap places. This will net you both achievements for both players fairly quickly.

It is much better to do this with a coop partner as relying on the AI to not die is annoying even on recruit difficulty.

NOTE: Your aggrometer wont always fill up. Try killing someone with the turret and it should shoot up or get off the turret and sometimes the bar fills up itself. If you struggle for kills try using a more accurate weapon or a shotgun for maximum damage.


Update: Due to the server closures I recommend doing this in splitscreen COOP or you can 'try' with the AI partner.

Best solution if you have an human partner is to start up "Aircraft Carrier". Enter the ship and you will get to a position with a riot shield. shoot out of the door to get the attention of the 6 or 7 enemies. kill one of them to get a full aggro meter, the other player runs now towards the enemies and kills them while in stealth mode. once all are dead kill your partner and start over from the last savepoint (at the riot shield).

cant confirm if it works if you just load up the last savepoint without killing your partner.