Get it. He never blocks it so you just need to learn at what points in time during the fight it is good to use so you don't get hit by him. You can also try to do a parry battle with him where you attack, parry his attack, attack, parry his attack, etc. A friend told me that's how he did it and it skyrockets his posture really quickly. I'm also going to try using the sugars. I haven't used them yet so I have some to burn.
Also, I've heard that having one of the big skills (like Shadow Rush) will help you with getting another skill tree later on in the game.