When the level begins, there is a partially-destroyed building in front of you. On the road behind this building is a Drop Shield that you can use to heal yourself.
You can use the Drop Shield in conjunction with an Energy Sword to kill any Elites that attempt enter the shield.
Clear the area of explosives to prevent deaths-by-explosion later on.
The turrets that spawn are invaluable. A common tactic with multiple players is to have one player pull a turret off it's stand, and fire at the enemies that try to come up the stairs, and have the other player man the turret and take advantage of the limitless ammunition.