I tried that probably 15 times, but at the start of my attempts blue girl was still level 1 as I was just following the story so had to level both up to 6 through failing and probably didn't have optimal enough talents to make the fight doable straight up even juggling. I beat it when I switched to bird girl as the big boss was trying to move in for an attack and she just got stuck endlessly chasing me in circles while blue girl got to hit her for free. Feels like you were meant to have 3 characters or like level 8 to do that fight.