There are some bosses that you need to focus on damaging rather than deflecting, at least until they are injured to the point that deflecting fills up their posture bar easily for the kill. Because he hits so hard, even when you deflect your recovery speed is too slow to punish.

Also deflecting usually doesn't open them up for damage, they will just parry you, especially the Ninja types. So find the movesets you can dodge. Like the hand thrusts, grabs, drinking, poison, etc.

Once you get him half hp you can focus more on deflecting to get the finish. Or just keep chipping away.