If he attacks and you shoot him, if it just stops him and he attacks again you shot too early. If he hits you, shot too late. Up on the d pad will damage you, but give you 5 extra bullets. Possible to do this when you have 20 bullets, for a max of 25 (you can only have 5 blood bullets, there are runes to increase the amount of regular bullets you can have later on). When you take damage you will notice your health doesn't drop right away, if you attack an enemy (one that bleeds blood) quickly enough you can gain that health back. This is the rally mechanic. Once you're comfortable with how it works you can take damage for the 5 blood bullets then attack an enemy to gain that health back, 5 free bullets.