She spawns 2 'a royal guardsman' around 40% (blue con, unmezzable, see invis, low hps). Her HPs are locked at 30% until you kill those guards.
If you wipe, they don't despawn and they do corpse camp. If the Princess regens to 100%, she won't spawn them when you get her to 40% again so be sure to kill the 2 guards that are still up from the previous attempt (before you re-engage her if possible) and then you can just kill her from 100%-0% without issue.