Here's another modification to the Jin method.
I find that the better I start the round, the more likely I am to finish it. Pretty much every character starts the round incredibly aggressively, so as soon as I see the "Rebel" announcement, I start mashing my counterattack move, which is either C or D attack (crazy sword or freeze). The crazy sword move is great for stopping combos, and freeze is great for heavy hitting follow-up.
Timing is crucial - chain your hits, if anything it just keeps them from doing anything. I don't like to immediately hit SP3 as soon as the heat gauge is full, I'd rather have it be at the end of my C mashing, which gives me time to set up the SP2 and C mashing a second time, keeping my constant attacks up as long as possible. My only exception to this personal rule was Ragna, that son of a .....
Assuming we are all familiar with Jin's attacks by now and have spammed SP2 to death--
Unlimited Rachel:
I like being closer to her than far away. Constant attacks up close keeps her from spamming her frogs and rods. I freeze her with plain D as often as I can so that I can interrupt her electric chair spamming. As soon as I freeze, I follow up with SP2 then immediately mash on C attack. If she jumps, then as soon as she almost lands I either freeze or SP3 if heat gauge is up. SP3 also gets followed up with SP2 then C mashing. SP2 is also a great way to fly over her frogs (which refuse to die).
Unlimited Hakumen:
As the round is about to start, I mash D to hopefully encounter his first attack (usually a combo). Freeze, SP2, mash C. He likes to jump a lot to land right in front of you which is handy for D once you know how to time it right. His counter attacks are a pain (DON'T hit him when his green or red shield is up). When he lands a hit, it usually starts a combo which is when I start mashing C again, which helps to counter. As soon as he jumps back a little to avoid getting hit, I either freeze or go straight to SP2 to mash C again, depending on how far away he is from me.
Unlimited Nu:
Honestly I've very rarely countered her first attack, and just do my best to recover. By insane luck I managed to have a perfect round against her ONCE (then she slaughtered me for two rounds). So since I know my chances are slim, I just go for the D mashing as the round starts. Much like Rachel, she is much more of a pain for me far away than up close. Her distortions can be avoided if you're lucky enough to be close to her at the time, as well as her normal projectiles. I freeze her as much as I can when I'm close, and in due time she is guaranteed to get me far away where she can spam projectiles to death, which is when I mash SP2 till I hit her, then go back to my C mashing. SP1 will counter some of her projectiles, and as soon as you see a distortion start, jump forward and hopefully escape death.
Unlimited Ragna:
I mash D as the round is about to start, and most of the time it'll counter his first attack, to where he falls back slightly. At this time (and not before) there are a few options. Either jump and SP2 to fly over him and land way on the other side, freeze if he is close enough, SP2 directly into him to land some more C mashing, or jump backwards to give yourself distance. As soon as he falls back he will most likely do his SP1 attack. I like to freeze him as the plain D freeze (NO d-pad) tends to make him bounce backwards which gives you some distance as well as give damage. This actually gives me enough distance to start SP1 spamming. When he gets close by running, I mash C attack to counter whatever hell he tries to unleash. If he tries to get close by jumping, then I freeze him to get him the hell away from me. SP1 like crazy when he's far away, and it'll actually counter his long range distortion. This is the one case where as soon as my heat gauge is up, I hit SP3 as soon as there is an opportunity since giving Ragna any extra time is suicidal. I've gotten him far enough away a couple of times where I spammed SP1 and it led straight to SP3. It was great when I had half my life left (more even) and he killed me with a distortion, and I didn't see it coming and so didn't hit the second controller in time.......
It was many hours before I even could beat him one round, then finally on a third round I miraculously got a perfect against him, hence beating the entire mode and as soon as I did I just wanted to lay down and cry.
Good luck to all, and I know I'm late in playing this game but hopefully this helps someone.