After you sit on Ares throne there is no way to play the prior levels unless you replay the game from the begining or you load from a previous save point. Ofcourse if you do either of these your powers will not be at the max as they may be (and probably will be) at the end of the game. What I would suggest is to play the "Challenges of the Gods" after you beat the game the first time. They are really good practice and you will be "forced" to learn more affective ways of battle. If you plan on beating the game in "God Mode" I would highly suggest you practice up on your techniques first. I will admit that the "Challenges of the Gods" are rather difficult, but that is what makes them so great. By being so hard to beat you must think of new ways to fight your enemies, this practice will make "God Mode" alot easier. Don't get flustrated if you have trouble with the challenges, try new button combinations. There are seemingly unlimited combinations you can do if you learn to use your attacks wisely. Once you feel secure with your "new found power" then it is always fun to beat the game again.

NO unless you replay the whole game again Iknow ive tried it sucks