You can also mitigate the doom effect with a barrier. It would then do (100% current HP - barrier) damage.

The trigger attack that deals 90% current HP damage is the first thing she does after she gets stunned. That does mean that on the same turn you beat her into stun with a normal attack or burst, she'll perform that trigger. If someone still had corruption leftover from the normal overdrive, it's possible for that to do enough to kill someone who's left really low on life after the trigger. I've also accidentally gotten one of my hime killed by the self damage part of a zeal effect (from summoning Yule Goat), because I didn't expect my burst to knock her into stun.