You need to cancel Hammer Fall, and in the first frames of that animation you do HPB. The way I do it, while I'm in Hammerfall, I do the motion of HPB and press Punch and Slash at the same time.
As Arafle said, during hammerfall you can input heavenly then press the punch button and slash button at the same time. This works because you can kara cancel hammerfall break into supers
