Army of Two: The 40th Day

jdmbt373 solution works very well (I used my partner on full aggro for quite a bit and the G36C upgraded), but I wanted to add at the end with the shotgun heavie and all of the enemies there is a easy way to beat them. The position where you step jump your partner right before the shotgun heavie can be used to shoot through the fence, even if it doesn't look like your bullets are penetrating. Kill all of the enemies through the fence to be safe until the shotgun heavie cutscene. Now back up for cover on the metal stairs behind you. (Make sure partner is set to stay close and not out running around getting downed) The heavie will come to the door to the fence and pretty much stand there. I opened up on him with the grenade launcher (did this after beating game), the heavie backs up a little and you can shoot him through the fence in the head. Once he is down send your partner in on full aggro to clear any left over enemies while you shoot through the fence. Once you hear its all clear head to the exit.

NOTES:
Also being thrown by one grenade does not count as a down, you actual have to be down to where you partner has to pick you up to have the achievement not unlock. If you partner goes down and you pick him up, it does not affect the achievement.

This achievement is pretty straight forward. start a game up on Mission 1, Chapter 1 - 'Shanghi Streets' on normal difficulty. go through the mission using cover and letting your partner get kills so you have to do less.. all you really have to worry about is the end where you have to go against the shotgun heavie. just make sure to aim for the head and let your partner do most of the work.. if you are having trouble with the shotgun heavie a good weapon to use is the AK-47 or the G36C upgraded...

i hope i could help!!!!!!