I watched one of my Auto-skill battles vs. Red Expert. If you have Maiden's Prayer equipped (4-turn CD) on Mordred (using Black Propaganda), the Maiden's Prayer will USUALLY go off right after the first Rage overdrive (but not always, because auto-skill randomly skips activating skills sometimes). If you're using an Advent Ragnarok-class team, she won't get a second chance to use the Instant Death debuff until about 40% HP. By that point your team will be ready to Burst and losing 1-2 kamihime shouldn't result in a defeat. Otherwise, her damage is nothing special and a team with even one healer should have no difficulty winning without casualties.
I suspect Blue Expert will be more problematic since you can't run a healer at all, and that DoT will cause serious damage if left unchecked. She also does more damage overall since she'll be spamming her team-wide AoE more often.