I would not try to get this achievement until you have finished the campaign, unlock the bonuses and upgraded the Decepticons.
I used Grindor for most of the missions. His deployable missle launcher really made a huge difference. Try to build up your overdrive as fast as possible by using different attacks and combos and use it when a lot of Autobots are around you. Try to save a full bar of overdrive for any bosses at the end of a mission.
For the racing missions, I use Sideways he's simply the fastest on the ground.
The number 1 key to getting all the platinum medals is knowing where all the time bonuses are. Take your time and search for them. Take a couple run throughs to make sure you know where they all are. Also, knowing where you have to go and getting there before the objective marker appears on your map really helps.
Experiment with all Decepticons and use which one works best for you, some work better than others depending on the mission. Good luck hope this helps.