Firstly, let me begin by saying the Decepticons are just awesome- always on the offensive never willing to give up. On the other hand, Autobots are more passive in their approach- reacting only when they should. Hence they always get their butts kicked-in the first half of the Transformers franchise.
So how would I defy the Autobots?
No no, let me start by saying I want to become Megatron, leader of the Decepticons, fearless, merciless, cunning and a true war tactician. What's more in the past two films he transforms into a tank and jet fighter. Really bad ass!
Ok, now lets move on to how would I destroy the world and the Autobots?
It's clear the only thing holding the Autobots together is Optimus Prime-supposedly the last desendant of the Primes.
Also with the Fallen now fallen, I am not required to obey anybody except myself, free to do as I please. So, instead of wasting time on the mumbo-jumbo trying to destroy earth by claiming its sun or looking for the missing shards, I would place all my focus on destroying Optimus the very source of the Autobots strength and determination.
But first I have to isolate Optimus first just like I did in Revenge of the Fallen. Undeniably, he is strong but not invincible as he already died by my hands once.
It's a matter of setting him up. And since Optimus is the only Autobot that can fly, I will lead him to outer space where Starscream and Soundwave will be waiting.
Far away from his pathetic Autobots the assault begins.
While crushing Optimus with my bare hands, I will order my non-flying combatants to destroy Optimus's loyal followers, Bumblebee, Ratchet and Ironhide before destroying the rest of the Autobots pests.
With Optimus and his precious Autobots turned to scrap metal, I shift focus on their human allies by attacking their defence systems and their leaders.
The world soon crumbles by itself as without a leader, humans are without hope and its only a matter of time before they start to turn to each other.
While the humans commence their own undoing, I'll travel to the moon to unravel the secrets hidden in Sentinel's Prime ship. With it, I shall be King of the Universe.
Having learnt the secret I shall return to earth and rejoice to its destruction. However, I will not wipe out humans. They instead will be my slaves. All of them!
Successful in my campaign, I'll hunt down Michael Bay and force him to make another Transformers movie. One where the Decepticons triumph over their puny Autobot brothers. Upon the movie's completion, I'll use my Ultimate Movie Card to buy my minions premier movie tickets so they can relive the glory.