The first thing I see is that the video is 10+ minutes. The second thing I see is that Peony has Poison Strike. When I got the end, I just let Peony work Heiðrún down to 1HP before sending Ratatoskr to finish the job. Would it have been easier to inherit a special on Peony? Good job on finding this solution.
I checked earlier and there was no problem. So please check again if your skills and status are the same as in the video. Thank you for commenting and watching!!😊