It will be a little harder, but it will save you the time of getting every soul just for an easy win. What I suggest is you get Mana Prisms, a lot of them. In Chaos's second stage, consistently use the Devil, Curly or Manticore Soul. Aim for the eyes on the corners first, as they shoot projectiles that can cause a bit of damage. Ignore the ones on the top, because they take too long and these souls cannot reach that high. Once you get the bottom eyes out, keep on using these souls against Chaos. Use Mana Prisms to restore your MP. After a minute or two, you should have beaten Chaos. I know there are many ways to beat him, this is the one I used before I obtained the Chaos Ring.