If you're really having issues that bad with it, you could always try getting your friends together for a LAN, though it requires two of everything (TV, 360, ODST, and network cable) in the same house, with a router.
Connect both 360s to your router, sign everybody into Xbox live, then switch your network in both Firefight lobbies from "Xbox live" to "System Link". You should now be able to get the achievement lag-free, barring any issues with your router.
It takes a little planning, but does make the experience less frustrating.
That's the best I can come up with. Sorry.