Getting the S ranks on hard mode without the obliterator gauge.
Some notes in general.
Use lots of Air Dash Kick -> Ground Dash Uppercuts (while jumping, hold down "B" and after you do air dash kick, you'll do ground dash uppercut).
Try to do special moves, like when an opponent is on the ground with a yellow "Y" symbol over his head, when you are surrounded by many enemies.
I use the Survivor Gauge (extends duration of Unlimted Mode) almost all the time. For the only hard episodes, 15 (hardest) & 17, I resorted to the Defender gauge. I also had to die alot to get good enough at those episodes for an S rank.
Watch this for some tips. You probably won't be able to be as good, or survive using thermoregulator gauge (like the guy in the video does) when you first try. Just use the Defender gauge until you get good at this episode.
Right as he is doing his second dashing attack towards you, jump and then do your Air Dash kick to then nail him. He is vulnerable from behind (as in if you can attack him right in his back then he can't counter you). Prepare for him to jump on you and press Y to counter. When he is knocked on the ground, pelt him with B hits until Yasha does his ending flip kick, and then do Y.
Try to go for A rank in time, but S rank in battle points & synchronization. I used the defender gauge.
Easiest thing to do is before you get to the mantra chamber (where Yasha talks about how they collected 7 trillion souls and converted them into mantra energy), don't hit burst right away. While you're at that wall that is shooting at you, hit Unlimited Mode and spam regular X attacks at the center of the circle until Unlimited Mode runs out = 999 battle points (max you can get in any stage is 999 from regular shots). In second rail shooting area where you fight three giant space ships, again hit unlimited mode and while that is on keep putting your cursor over the faces of the enemy ships and spam lock-on shots. Just sweep your cursor over all three faces, and spam lock-on attacks. Doing each of these just takes about an extra 20 seconds each time, but you get a ton of points.
When you are fighting Deus, when you get close to him he'll use his lightning nunchucks for one series of attacks, then a steps forward to make another series of attacks with nunchucks, then he'll pause and do something. Right after he does that something, is when he is vulnerable. Jump up sideways circling around him and when he does that something, perform your air-dash attack to get close to him, and hold button down so then you do ground dash attack = get close to him while he's vulnerable.
Some of the "somethings" he can do after two series of nunchuck attacks are a big spin lightning kick, a line of lightning strikes that kind of spirals out from him, some sort of dash attack, Jump into air then slam into the ground (while as Yasha) and probably some other stuff too. Sometimes while he finishes this "something" move, he'll dash away somewhere else, but he'll still be vulnerable for a bit after he dashes away.