Mordred's main Artifice is her Ending Act - note that as it occurs at the end of the turn, her 50% reflect lasts effectively one turn only, but is permanently active unless removed.
As that reflect prevents you from effectively dealing damage to her, copious amounts of dispel or a reduced team size to allow the dispel act to be used more frequently are necessary here. The CR support girl for this boss is Stage Girl Hikari, who has a dispel attached to a 176% multiplier attack. Note that moon attackers who dispel after attacking are not recommended as they will kill themselves.
1) Challenge Revue Support: Strength Futaba
One of the main established ways of clearing this boss is by using Strength's counterheal and climax regeneration to take all damage and keep herself alive (some unit skill and/or a birthday memoir recommended for the stat boost). Incorporate a moon damager such as Hermit Mahiru, but avoid attacking Mordred when reflect is up.
A two-girl team involving strength + a damager with a pre-damage dispel on normal acts, such as Shuten Doji or Kaguya Tamao works, but is very slow.