Trulon: The Shadow Engine

This is not too difficult, though you will want to be careful not to go TOO fast. If you skip too many battles, you'll be woefully under leveled for certain parts and find yourself dying quickly and doing almost no damage. However, you can speed everything up by skipping side-quests, all dialogue, battle animations, and any overworld encounters. Doing all of that, you should still be able to do the main story missions and fight pretty much every battle and still complete the game with plenty of time to spare. Here is a mini-walkthrough that gives you some time goals for certain parts of the story.