Explanation of why this kill is RNG:
If he walks backwards while blocking, the next attack is a gamble.
If you keep strafing, one of his diagonal slashes can hit, or in p2 tail swipe.
If you walk backwards, he can catch you with certain attacks if he does them immediately.
If you attack his shield, he can drop it to do an attack immediately. Even if you do hit his shield, it's not guaranteed he'll do the shield bash, sometimes he'll just do a different attack which you're not in position to dodge.
In phase 2, the main gapclose combo I'm looking for from him is to do his running horizontal, into a thrust, which I will duck under using my weapon's R2. It's slightly inconsistent. There's some other combos I explored gapclosing on, but I never figured out if they actually worked fully.
What you actually do see in the video is a strategy where I outspace his diving thrust and bait out the small tailwhip through positioning and then strafing it. This is a new strategy I developed, as normally you don't want to be anywhere near him after the dive due to his MASSIVE double tailwhip.