You can't beat the game at that time limit, unless you skipped all cutscenes, one-hit killed bosses with Quickening chains (30 or more, mind you), and had a perfect enemy distribution. The MAX+Active battle speed would, if anything, help your challenge go quicker, as long as no one dies or gets hit majorly to require mass Healing.

Also, since it's twenty hours, you won't be able to evenly level your characters to the point that you could get all your Quickenings or high enough levels as fast as you can to endure running through all the areas. Level grinding is out of the question.

Without Gambits, I'm clocking this challenge to be 26 hours or more. 3 less with. You need another five or ten hours if you want even an inch of leeway. However, if you try it, by all means, go ahead. Let us know how you do.