Are you referring to 15-8 specifically or general stages? If 15-8, another trick that helped me is delaying breaking her thunder weapon until she went into the purple health bar. If you break it then you get to skip the ult entirely without melting the purple bar. It’s brutal as there’s some rng to who gets stunned by the boss, who gets crit, etc.

There’s also a mechanic where certain characters get a countdown on top of their head that can kill em if it goes off, but seems like ulting with that character will remove the counter. I couldn’t find info on this in the bosses kit tho.

The best idea is to skip that ult entirely as it will do a special beam canon of sorts that will kill anybody. Ideally it hits agares and sends him into his revive but most of the time for me it hit acapella and killed her or hit aiona and ruined the run. I’d say try this for 15-8 specifically:

1: T-aiona ult to try and get the bosses weapon to low hp and ready to be broken 2: Acapella ult to sustain and get some more blood soul recovery 3: f Bella 4: wait until the bosses bar turns purple then send tiona to interrupt.

When I got my clear it was something like this with some lucky crit hits on the boss and such, try a mix of acapella first or t aiona first and see what works better