I made a date mistake and we're actually getting the 70% light eidolon in a couple weeks with the union event. The wiki for some reason had the specific girl I was looking at placed on the bottom instead of after Quetzalcoatl.
Anyways, that 70% is actually its no break passive. It goes up to 90%. So it's quite good as long as you can keep you main team alive but whether you can actually fully break her or not may not be up to you. Hopefully you have a active union.