Gears of War 3 Game Guide

Act Five: Chapter 6 – Reckoning

The fight will be between Lambent Drudges and Theron Guards, and they don't
like each other nor your squad. Really you just need to sit back and not die
as all the NPCs will take care of each other. A scene will play and then you
can help finish off every enemy, but feel free to let them harm each other for
a while before mopping up.

The final battle with Tempest will start by just shooting its mouth and 
keep any cover between you and its mouth when it prepares to shoot a beam. It
can move to the side, so keep eyes on it at all times.

When it falls and the Hammers of Dawn pop up, keep your lancer and pick one 
up. Hold the sight steady and fire about six shots at it as Shriekers should
be inbound. Once Tempest flies away, take cover and kill any minions you see,
but watch for Tempest to pop over the edge and shoot beams from a few spots. 
You are prompted each time it appears, or just stay behind one of the hammer
columns. Once it is back in the air and shooting beams at you, hit its mouth
with the lancer. You will down it and hammer it again, kill more Shriekers, and
then hit it in the mouth one more time and a scene will trigger.

*NOTE: Ammo boxes respawn by the structure for more lancer ammo. The hammers
seem to respawn too, but try not to reload one unless you're out of the one
you have. You can use the structure as cover if it flies to a side.* 

Therons will appear as you hammer Tempest again. After it flies again, hammer
it once more to trigger a scene. When it climbs the spire, aim low to hit its
back even when it moves and it will fly away soon. Grab some ammo and it will
fly around again, and watch for Therons. Bring it down and hammer it yet again,
and from here on out you need to really keep the Therons in check as you use
the hammer and bust it off the spire. There isn't much longer to go, so just
keep alive, use the time while Tempest is down to kill Therons while also 
using the hammer, and use some of the spire time to kill Therons as well.

Enjoy the final scene and what should be the end of the Gears series.