Halo 2/Cairo Station

Easy
The first thing you should do for this level is the obvious. Go down the stairs, and you will find various weapons. They are the: Battle Rifle, SMG (dual-wieldable), and the M6C Magnum. When you walk over or close to these weapons, you will pick them up. Once you've chosen your weapons, walk over to your marines, and they will go through the next door to your first fight.

Walk down the stairs directly in front of you, down to where Sergeant Johnson is manning a turret. The door you see across the room is where your enemies will come from.

Home Field Advantage
After the door blows up you will fight several waves of Covenant. Then head through the door and to the right and fight a few more Covenant. Head up the stairs and either head down the stairs across from you to fight several elites or jump down the window to the left with a dead marine in front of it to join 2 allies in a big room.