Transformers: Revenge of the Fallen

All you really have to do is keep playing the final mission over and over again. You get an easy 200,000 E just for killing the boss. And once you start getting the level down you can easly beat the mission in under 4 minutes. So easy Plat. Medal as well. Hope this helps.

The most time effective way to earn Energon in this game is to head over to; Cairo Pyramids - Defeat The Fallen.

And defeat The Fallen over and over again, once you've gotten used to the level and what you need to do to finish it you should be able to finish the level in about 3 minutes, which not only will net you the Platinum medal for this level but for defeating The Fallen you'll net yourself 200,000 Energon which is what is used as currency in this game.

It's worth pointing out that this achievement is accumulative, you don't need to have 3 million Energon at any one point to unlock this, so I suggest that while racking up the Energon you work on this; Solution for Spoils of War In Transformers: Revenge of the Fallen, doing both achievements at the same time will make completing the level that bit easier/faster.

I've linked in a video of someone playing/completing the last mission (named above), if you find yourself stuck on it or confused about what to do, use it as a reference!

* ^{Credit to yanceygrc54 for the video}

Once you finish the Decepticon campaign you can replay missions. Simply repeat the last mission over and over again until you earn the remaining energon needed.

Just play through the Decepticon campaign, completing as many secondary objectives as you can.

If it doesn't unlock by the time you complete the campaign, simply replay levels until you get it (which you may well need to do to get Platinum medals in all missions anyway).