I mean, she's just a clearly bad player.
Her strategy was "well, if I kill their leader, surely everyone else will just give up and die immediately "
Then she chooses to panic and spam random AoE's instead of just blitzing the healer
Either the author themselves is bad at RPGs or she's written to be bad at them
You're ignoring that she couldn't see the battlefield due to a passive debuff she's had since chapter 1. She's "spamming AOEs" because she can't actually see around her, and the Sunlight itself is acting as a constant debuff due to a second passive trait she's also had since chapter 1.
You're also ignoring that taking out the one coordinating all the others
is a sound strategy, because if they happen to actually be the only one with a plan, that throws everyone else into chaos and confusion.
And finally, she's
one player character against a team of dozens, who are actually coordinating for this fight and planned ahead with strategies. She went after the one seeming to lead, in the hopes that doing so would dissolve their cohesion, but she was always disadvantaged going into this fight.
The one thing she really got wrong was not being level-headed, and not running once things started turning bad for her. She got cocky, because she is objectively much more powerful than any of the other players.
But again, they had strategy and numbers and synergistic cohesion on their side, and she did not.
I'd suggest reading back through. It's all explained why it turned out this way, and none of it is because Rare's an idiot or a bad player, outside of her hubris.