Once she divides, attack her normally.
Luka will kill one of the clones, and she will say something along the lines of "blah blah I can regenerate lolololol", at which point Salamander will appear, and tell you to summon her, and that you can control her power now.
Summon Salamander and use the newly improved Chaos Star.
I'm actually surprised you made it to the end of the fight without knowing how to beat Heaven's Prison

...
She has 2 grab attacks. One of them, "Heaven's Prison", you have to ATTACK to get out of it. 2 or 3 times in a row.
The other one you can struggle out of.