You need to complete all 4 DLC Episodes on Hard, and get an S Rank on all the missions. I highly suggest you already have the "Obliterator" Gauge unlocked (You will unlock the Obliterator when you complete the game with the Mortal Gauge), as the Gauge infinitely enables the LT Burst, which makes you nearly invincible and gives you unlimited Y attacks. Try to get an overall Synchronic Rate (Which means Quick Time Events) above 90%, so you'll get an S for that on every mission.
Now onto the Guide:
Chapter IV: Nirvana
Warning: SPOILERS AHEAD!
Episode 19: The One Behind The Curtain
When fighting Chakravarta (The Buddha Boss) you need to use alot of B attacks, and when he goes down keep attacking him with B for a couple of seconds, and then use Y. Keep doing this until you fill up your first Burst Gauge (RT). Don't use it yet, keep fighting for about 10 seconds then use it. Same applies for the second battle, but once you fill up your second Burst Gauge, keep fighting him for about 20 seconds.
Episode 20: The Key To Victory
You'll find yourself fighting as Yasha, and you need to defeat alot of Gohma, knock them down with the "B Dash" (Hold B) and then use the Y prompt. Keep doing this until almost every Gohma is dead. If you fill up your first Burst Gauge keep on going for around 15-30 seconds. Then they will spawn a Turtoise (A big turtle) and a couple of those flying fish thingies, try to focus your attention at the Turtoise, and once he starts spinning around (Which he does pretty regularly), kill the flying enemies. Once you fill up your second Burst Gauge, keep on fighting him for atleast 30 seconds. This should do the trick.
Episode 21: A Rude Awakening
In this Episode you will be fighting Yasha, so expect some hard parts if you don't have the Obliterator. Just counterattack all the attacks Yasha throws at you, and spam the B button until he is laying on the floor. Bash B a couple more times and execute him with Y. Once you fill up your first Burst Gauge, keep attacking him for about 20 seconds. Then the next battle will take place in the air, while falling down. This wasn't very hard, just counterattack and watch Yasha's moves, and dodge if you have to. Once you fill your second Burst Gauge take him down 2 more times (So attack him until he is executable, use the Y prompt and repeat) For the third Burst Gauge, repeat the method of Burst 1, but instead of attacking him for 20 seconds, just execute him once more.
Episode 22: A Life Well Lived
This Episode takes quite some time and a huge spamload of QTE's, so always be on your guard. The first battle takes place in space, and I already had the Burst Gauge full before I could use it, so activate it as soon as possible. For the second Burst Gauge, keep attacking him for 10 seconds, once it's full. Burst Gauge 3 can be activated normally, no need to attack him any further. Burst Gauge 4 is activated in a cutscene, and once you fill Burst Gauge 5, finish him once more. The following Burst Gauge's (6, 7, 8 and 9) will happen in a cutscene. Expect alot of QTE's, but note that you'll also see the QTE's of Chakravarta (They're Japanese), but those are NOT meant for you. It's Chakravarta who needs to activate them, so just ignore them. Then for the final Burst Gauge, you will face the biggest B Spam of the entire game, which takes about 10-20 seconds. Once you have it filled, use it and finish the job.
If you still can't pass it using this Guide, please tell me what you did to make it happen in a Comment, so I can edit my Solution and make it better. Good luck.