A little preface on how boss fights work in Crystar, there are two categories all bosses fall under.
1. Humanoid
2. Abberant.
Humanoids can be interrupted any time they're not attacking and actually flinch, letting you launch and combo them properly, but in exchange they typically have much more dangerous attacks that come out much more quickly than an Abberant type.
On the other hand, Abberant types tend to be more durable and don't react to damage while their attacks cover large parts of the battlefield. However, Abberants also have a lot more wind-up which makes them easier to deal with.
All bosses start off with a small pool of fairly non-threatening attacks, and either get newer, stronger attacks, or upgrade their old ones every third of their health bar until its over.
The boss I'm here to talk about,Sen Megumiba is an example of a humanoid type. The first third of the fight is pretty much just a taste of what's to come. Their ranged attacks aren't very threatening, however they do have one melee attack that comes out instantly and does a decent amount of damage.
Second third of the fight, said melee attack gets upgraded to one that hits 6 times. With no wind-up, and does enough damage to wipe out Rei's health bar in an instant. They also add a spinning slash attack that comes out pretty quickly, but usually isn't fatal.
Finally, in the third and last phase they add two new moves. One ranged attack that launches 7 fast-moving homing swords which can only be dodged by running in close to them. And a second melee-attack that not only comes out instantly, but launches you into the air and is usually followed up by a spinning slash from phase 2 to finish off your health bar.
All this isn't too bad on its own, since the game has an auto-heal function that'll use healing items whenever you're too low. However, they also come at the end of a chain of 3 bosses, the previous two of whom being abberant types with one almost impossible to dodge attack each that's available from the start, so even if you are taking advantage of the auto-item use you're likely somewhat drained. They're also the second boss in this chain you have to fight using Rei alone, whose well-balanced stats tend to lead to her being more vulnerable than the others.