For this achievement you have to complete the "Road To Ruin"
DLC in co-op, it doesn't matter which character you play as or which difficulty you play on.
Now the deleted scene can play out in 2 different ways, you can either choose to go in all guns blazing, for a guide for that option please see here; Solution for Highway to Hell in Gears of War 2
or you can choose to try and go through undetected, for a guide for that option please see here; Solution for My Way or the Highway in Gears of War 2
The way in which you choose to playthrough the deleted scene really doesn't matter for this achievement, just choose whichever one you want and get through it, refer to the linked the solutions if you need any help!