Transformers: Revenge of the Fallen

The Fallen is the last boss of the game, you have to destroy him in under to pass; Cairo Pyramids - Rise Of The Fallen

Overall it's one of the easier missions in the game, you're using the powered-up version of Optimus Prime so you can fly about now which makes reaching him infinitely easier.

When you first start he'll be standing atop the pyramid in front of you, I find it best to fly up toward him and just lay into him with one of your weapons and switch when it's close to overheating, after he takes a bit of damage he'll teleport away from the pyramid and begin using a column of fire attack, just keep moving in one direction and you'll avoid this without even having to really try, continue to unload on him with both your weapons, again switching just before one is about to overheat.

After you damage him to a set amount he'll vanish and a few Decepticons will turn up, just lay waste to these guys from the air, there's only a few waves of them and they're no real threat.

Finally The Fallen will reappear atop the pyramid, this is the homestretch now, keep firing at him and once you get his health bar practically all the way down you'll trigger a short cut scene of Optimus Prime defeating The Fallen once and for all!

I've linked in some videos showing you how to beat The Fallen, use it to learn the level layout and enemy locations/attacks, doing so may come in useful if you're stuck on this mission!

* ^{Credit to captainxena for the videos}

This fight goes in a loop, I'll explain each part as needed. he seems to trigger each part based on his health bar, but even with a maxed out Prime, it can take a few loops (Megatron feels far more powerful, especially with his fusion cannon), a bonus of Prime is his shield to absorb some of the damage, use it whenever your getting low.. so anyway...

Part A - the Fallen is at the top of the pyramid, just fly up, and use your primary on him, swap when ever you overheat, but the secondary didn't seem as effective to me. after a few hits he'll teleport to a new side, get around to him avoiding the debris, until he triggers a 'cutscene'

Part B - He will then start using a fire column attack, which is easy to avoid fly flying to the side, he'll come from a distance, so get sight of him and fire at him as he comes towards, he'll teleport away for another go or two, then another 'cutscene'

Part C - as Sam says, a bunch of enemies will turn up, keep away from the 'Long Haul' & ones with rocket launchers, as they have a (bumblebee's) stun attack - although you can just blast them from the air, when they're all done you'll get your next 'cutscene'..
note: If you spot the Fallen on the ground on one of the paths, blast him

Which leads back to part A, followed by B, C and repeats until..
When the Fallen is low on health & triggers part C, this time, his way point will still be active, as he is at the top of the pyramid (just like in part A - So I'd just concentrate on the Fallen, keeping an eye out for any plane Decepticons, it's usually only one at a time, take them out if your getting low on health.