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This achievement requires you to obtain Platinum medals on ALL Decepticon mission.
Do NOT try this Achievement until you have fully upgraded the Decepticons and until you have finished the story...
For most of the missions i used Grindor, Longhaul and Megatron as they are the strongest. For most i would recommend Grindor as he has a fast Transformation and deploys quickly, but if you are going for missions that require raw power to complete, then Longhaul and Megatron are your guys.
Megatrons Beam Cannon (Pressing Y) will deal with larger enemies very quickly in the missions where you have to defeat tough bosses.
I did however find Starscream useful in some missions as he is very quick (and even though he is weak) he is quite powerful. The only problem with starscream you have to sometimes hang back a while.
If you get stuck just experiment with characters, use the ones that suit you best and don't give up this achievement isn't all that hard.
For the racing missions i used Sideways... he HAS to be used as he is by far the fastest vehicle. Alot of people had trouble with 'The Shanghai Spiral Highway' mission...
Below is the Youtube video which i refered to and after a few watches it got me the Platinum first try.
Good luck guys i hope this guide helps.