Army of Two

i did this on the first level, just shoot your partner until he gets downed then heal him, easier with a person partner rather than the AI cuz the AI will shoot back at you

or you can just play the game until your partner goes down, either way, super easy

With a human partner in co-op, this is very easy, since they can just stand still and let you use a grenade on them for a quick knock down so they can be revived. However, when playing solo, your AI partner will shoot you back if you shoot them too much. In that case: A little into the first level, you will come across an area with multiple cars, trucks and concrete barricades to use as cover. Several enemies will shoot at you. This is a good place to get this achievement. Send your AI partner into aggressive mode using . You may have to regroup him with first if he's still at the previous area. This will focus fire on him, as indicated by the Aggro meter siding towards him. Depending on the conditions, the AI may be able to down him. In this downed state, he can't shoot you back and can now be dragged out of the fight and revived.