I usually choose Ali for this.
I found this quite easy when I realised that if you start leaning one way and then once the bag has passed you can lean the other way and back (i.e. two leans) before the bag returns.
If you find that you are on an odd number of leans left, you can quickly flick the Left stick an extra time and perform three leans.
This will transition you to a much easier two lean per bag pass strategy.
When you are asked to punch, wait until the bag is passing and the press square. This will send out a jab and allow the boxer to return to a lean state before the bag returns.
When you get cocky, you can repeat the three lean strategy and rack up loads of points. Two leans + the occaisional three lean will net you the champion rating.