First of all I would not worry too much on getting all the Platinum medals until you have beaten the campaign, unlocked most of the bonuses (so you can use any autobot you want for missions) and upgraded your autobots, since upgrades like faster cooldown and more turbo will really help you shave time on missions.
Next I found primarily for just about every mission I used either Iron hide or Breakaway. I found Iron hide was the best through ploughing through many enemies quickly and Breakaway is good for quickly taking down bosses or special enemies with headshots from his sniper rifle. Ultimately though I would recommend Iron hide since he tends to get things done faster overall.
Launching grenades at everything that moves tends to get the job done and switch to iron hide's shotgun only for especially strong enemies at close range, but don't even waste time trying to shotgun small, fast enemies. When you are not fighting remember to use vehicle mode for getting around, always boosting or using advanced jump to get around obstacles. Also using his advanced melee attack by attacking them from vehicle form takes out small decepticons quickly and adds to your overdrive meter.
Ironhide's overdrive and special ability will be a big help in taking out enemies quickly. Always make sure you have Ironhide's missile launcher deployed, it has great range and aim and will kill enemies you did not even notice were there. When you go into overdrive mode so does the missile launcher and it will kill pretty much everything in the area before you even knew it was there.
Some of the bosses you fight run away from location to location as you fight them and you cannot damage them as they run away, so missions like this are more dependent on how quickly you killed the decepticons before the boss showed up.
For me the hardest mission was the one where I had to "escort Iron hide" and instead of breakaway I chose ratchet. Make sure you hit all 5 of the time bonus icons and don't stray too far away from Iron hide or he will just stand still. I used advanced melee and my machine gun to take out smaller deceptions and when the helicopters landed I would plant 3-4 sticky grenades on them.
Overall most of the levels aren't too bad for making the platinum times, a big part of it is learning where the time bonuses are, where you have to go and getting there before the objective marker tells you to, and killing enemies as fast as possible. Finally use whatever autobot you are best with, if you can kill things faster as bumblebee than you can Ironhide then go for it.