I wouldn't call it an issue with not downing her fast enough
That's exactly what it is. She'll do the Charm 3 (maybe 4) times, and then that's it, you lose. My FC calls her the flying Demon Wall. I imagine if the party member she eats has low HP it won't kill you, but most healers are not going to let that happen. You need to DPS her down more quickly and she'll never use this.

If you want to run a trial on the HP theory, first we need to find out if it's 3 or 4 times, this has only happened to one of my groups, and it was when it was first released, so I have no idea. Then, on her last time, need to leave your HP low, hope to hell she doesn't eat the healer, then *spam* heal when she eats the party member and see if it matters.