Just a heads up for you. You don't actually have to play someone online. If you have two controllers just go into multiplayer and start the game. I loaded my brothers profile (not a live profile on my xbox) and started the game. We both got credit for the ach and I never even played. As soon as the ach unlocked I quit the game. Hope it helps you all and saves you some time.
Started Using x360a.org - June 2008 - 3K Gamerscore
Hit 100,000 GS on 12/22/2011 with Mile High Club - thanks to all the boosters and this great site.
Looking to Boost: Halo ODST - Endure Only, Ghostbusters, Quantum of Solace, GTA IV Multiplayer, Halo Reach DLC
Message me on XBox Live if you need an extra player for any of those.