Army of TWO The Devils Cartel

A Good place to get this with fairly little effort is in Narco Hell 34. A ray of sun.

Just as you enter the room with the large green skull outside of the door when you are making your way to the roof there are 3 men standing with their backs to you.
Run through the door and stab one in the back then reload the checkpoint which is right outside the door

(If you have a full overkill bar upon reaching this point you can get 2 and sometimes even all three enemies with the backstab making this much easier)

Perform 50 Backstabs

now to do this the easy way
you need to go to the mission narco hell - chapter mad science
and fight your way through until you get to a dead end where you have to shoot a blue barrel and create a hole in the floor

the next area will have rooms with alot of blue barrels and will have guys with machetes running towards you

when they come, run away so that they focus on your partner then come back and backstab them

sometimes there's two of them coming, quickly shoot one with your handgun then run away

when you reach the vent that you have to move..reload the checkpoint and do it again until you get the achievement


Additional info Posted by Healtti
Reloading checkpoints causes the game to NOT rack up your stats, viewed through Main Menu -> TWO profile. However, it still counts towards the achievement. Don't get discouraged, trust the guide.

Perform 50 Back Stabs - 10G

The other solutions are spot on but I think there is a quicker way and a more fun way if you love OVERKILL!

If you haven't got the 50 back stabs by the end of the campaign then load up the Overkill contract on the level Rally Point.

On this contract you will be in permanent overkill mode so you will be untouchable.

What you want to do is have your partner draw the attention away from you allowing you to run up behind the enemies and back stab them! Using this technique I managed to get between 10 and 15 back stabs a contract. Just rinse and repeat and it should only take a few goes and will also help you towards the Blood Sport achievement.

I hope this helps :-)

If you have the overkill edition or bought the overkillers add on (I got fed up of grinding 2k per 2 minutes and just bought the overkill contracts add on) then load up the overkillers contract for the mission outside contact and kill your way through until you are told to kill the brute. Kill him, then stand where you encountered him and kill enemies to keep overkill going, stand back a bit (on the mouth of the entrance to this area is fine).

Every minute or so one enemy with a machete will yell "argh" and rush bravo, when you hear this happen run the 10 or so feet to the enemy and get behind him, (he never turns away from bravo) backstab him. Repeat as many times as needed until you get the achievement, the enemies here keep re-spawning too so you can rack up 100K+ easily in 20 minutes for the cash to buy things.

You can easily get this in one session like this as well as lots of cash.