This achievement is for clearing a stage of Invasion on XBOX LIVE (Invasion=Hoarde Mode in GoW).
As the TA ration suggests, this is damn hard, and not just for clearing 1 level, or 5 level section. When they say "stage", they mean waves 1-50.
The main problem here is finding players, as the number of times I have had co-op players to play with is very rare, and unless you have 3-4 in your party you have no chance. You can play on your own but the difficulty level is very tough, and even the basic green soldiers tear you apart if caught out in the open.
Probably one you are going to have to get via a boosting session (as you'll never get a 4 player group together otherwise imo). Good luck!