Each time bull hits the wall while charging it triggers rocks to fall. For stage 2 and 3 of bull you should see markers at top of screen indicating where the rocks will fall from
The longer your combo the more damage you deal. So it makes a lot of difference if you are closer to the bull when his charge is about to end.
Once you have dodged the bull, Try chasing bull while he is charging away so you end up closer for more time to deal damage.
Also bull charge is triggered only when you are close to it, so you can be on a ledge to take a breather.
Stage 3 of bull, you need to be on highground to dodge the beam, also you need to hit the crystal which is tethered to bull when he is down to deal damage (bull is invulnerable to damage in stage 3 directly)