Gears of War 3 - Reckoning

Once you are on the roof, just start shooting. There are lambent and locust everywhere but you have decent cover and they will fight each other too. Once they are clear, you will have a different problem to deal with. As the rule of thumb goes, aim for the mouth. In between blasts, get behind cover. The cover of the outer ring of the roof will be damaged and eventually be destroyed but the inner ring of cover won't be destroyed. Eventually you will be told to use the Hammer of Dawn. Use the Hammer while she's down on the ground. After taking too much damage she will run and attack from the side of the building. Stay away from her at this point. When she starts flying, attack its mouth. Rinse and repeat.

After a few rounds, Tempest will climb up onto the machine and try to destroy it. Its back plates are open and you can shoot it. Eventually you will knock it off. Then you just need to shoot it in the mouth and hit it with the hammer of dawn one more time...