All credit goes to:
'MrPTClassixX' on YouTube for making this beautiful video.
This guide uses the unlimited magic skin (cow) which is unlocked by completing the Challenge of the Gods. The video shows how to beat the story in under 4 hours (you have 5 hours for the achievement) without any glitches, despawns or commentary. Just you, the cutscenes (which don't count towards the time) and your unlimited magic.
My number 1 tip is to spam magic and check your time by saving at a save point, when you view your save points it will tell you your time that you have taken so far, remember... 5 hours!!