Here's a link that pointed out the mechanic for the second part of the fight. https://ffxiv.consolegameswiki.com/w..._for_One_World
She'll ask for your aether twice, then when she says a about holding the line, that when you should click on the "bubble" that surrounds her or the "blade" if you can still target it.

I waited for a third time for her to ask for my aether and when it didn't come and she kept sayin hold the line and the enemy's aether line was almost full, that's when I clicked on the "bubble" and completed the fight. Hope this helps.