Yes but since we are obviously taking these characters from their most powerful states and pitting them against each other, Shulk is pretty much impossible to beat at his most powerful point. To the best of our knowledge, he is omnipotent at his highest point, having been given the power of the True Monado, the power to destroy and recreate the universe at will. Defeating him at that highest point is, from all practical points, impossible.
Not really sure. Kratos killed Zeus after willing himself back to life. Doesn't really matter how many times Shulk destorys the Universe if Kratos can will himself back. Also, being able to freeze/reverse time really nulifies anything Shulk does... unless Shulk exists outside of time itself in which case it doesn't really matter because doing so would mean Shulk doesn't exist... (a common paradox/pitfall built into omnipotence) lol.