A small update.

Boss does have increased base def, but the value is 13 instead of 15.

The value of her self-inflicted def down is 23%, which actually brings her active defense to roughly 10. Guess is that her raging overdrive requires some level of debuff to deal damage, so the permanent def down makes sure she at least always does 1.5k worth. However, that only means the lowest you can bring her def down to is 6.5. I'm still not sure of what frame the def down is applied on (maybe stacking or weapon/eido frame. Or just a special new frame on it's own), but your A-C frames should all stack above the current def break.

Also... I'm noticing that atk break does actually affect the scaling of her raging overdrive. Can anyone else help to confirm?