phewwwww, just got it, all i did was....
i completed the whole game as co-op host on realistic = no acheivement
i then completed 5 scenes as co-op partner (any scenes), then up it came = acheivement unlocked 75g
Now, im sure this wont work for all off you, but until we can put our finger on what actually does trigger the acheivement try this. It worked for me.