BT Asura and Normal Asura are both the same input.
When you are in BT, Just do Asura as if you weren't and it'll come out fine. There's not much need to "practice" it.
ExM: It depends on how your 6AAA hits the opponent, if you time it right it's guaranteed.
If you don't like using 6AAA as your ender, In situations when you can 6AAA. Just use 4A+B.
This will do several things.
A. Hit them on the ground, this does more damage than if you were to 6AAA. Off a close 22B this is like 70 damage IIRC.
B. Hit them while they are rolling, some people might try to roll behind you, they'll get hit.
C. They tech and eat more damage (Teching will result in resetting the combo scaling, but off a 22B the scaling is already dead)
D. They roll and block. Probably their best option, saves a little Soul Gauge lol
E. They 4G to block it, eat soul gauge damage and 4A+B's killer mixup.