At first run around and kill the people, buy a SMG to deal with the rounds.

Now when you have enough money go to the actual 'dome' part and go up the ladder on to the platform. Both buy a striker and a secondary weapon. Also buy one grenade launcher and a sentry gun. Now hide behind the corregated iron, this will protect you from other fire. Place your sentry, not immediately by the ladder but a few feet away from it, do the same with the grenade launcher but with the other ladder.

Between the rounds buy a spare sentry and grenade launcher. When it comes to choppers go inside the building under the dome bit and shoot-take cover-shoot-take cover. Have one person kill the other people coming towards you and the other shooting down the choppers.

When it comes to juggernauts, kill them with predator missiles.

Doing this will hopefully get you to high rounds!