This achievement must be done in online multiplayer; you can't do this in local online versus play. You can 'spot' players by clicking (LS) while aiming at them. This allows your fellow teammates to see their whereabouts, because an icon will show above their head, for a short time. This feature was not implemented in the original Gears of War.
Spot enemy players and hope that spot results in kill by a teammate. It is recommended you use spots at choke points where multiple players congregate and shotguns are being used then spot as many enemy players as you can. You can only spot one enemy player at any given time.