Hey there! Yeah the index are one tricky trial.
The boss is a hard one because you are open to attacks every now and then.
If you're thinking about beating em I recommend moving back and forth to delay Glorias attack (the one that stabs you) and using one skill at a time rather than a combo.
Using a character with movement skills can help pretty well too.
Shi assoc fixer is a good one, it's first skill can strike past the opponent making it great to attack and distance out of the triple gang.
Thanks for playing the game and hope you have fun!