This is very simple.
In the main menu , choose to play co-op. Now press X for a random matchmaking or press Y to invite a friend.
There are 5 ( five) diferent missions. Just beat all five .
I DID WITH THE SAME PARTNER AND DIDN´T GET GOLD STAR in any mission, and still got the achievement.
A good communication is the key .
Try to save your partner when he´s grabed by the yellow flood !
Well, i used most of the time the AR (assault rifle) and the shotgun for weapons. And the overshield for armor hability.
One last thing, when your partner dies, active your OVERSHIELD until he respawns ! This will save you !