Okay so I was following the guide above and was getting really frustrated because I couldn't get past round 15. A few things to add to the x DJ Jez x's guide;
To get to this game type go to multiplayer-xbox live-private match-game setup-mode-alternative-safeguard.
On that same screen click options and then turn health to double and regeneration to fast as that will make this infinity easier.
When possible always stay up on that ledge, get the shotgun since this is point blank, stick as close to that window as you can without popping out from behind the sign, prioritize getting helo scouts and the sentry gun( when you're low on health/getting overrun/fall of sign then call it in and you'll be teleported with full health into a helicopter with a one hit kill sniper. Make sure to position yourself near the building for when it drops you off), hold on to the ghost teammate you start off with so if you die you can just call him in and get revived and keep in mind you only get one of him, lastly never stray too far from the building for and if you do just get back before the next wave starts.
Make sure to get all the different perks, especially Marathon and fast reload(or whatever it's called).
Sorry about the poor organization of this guide but I don't think anyone actually cares. Hopefully this helps you, it sure would have helped me.