Her main artifice is, of course, that nasty opening act. Her secondary artifices are the heavy multihits on her normal attacks, and the Ending Act which slowly builds her damage. She also gains 50 brilliance every turn.
She is immune to burn and poison, and is Special-type.
The CR support girl released specifically for Percival is Vivian (Lady of the Lake) Junna, who gives an 80% wind damage reduction to her team as a passive. Other than that, using large amounts of bulk US (similar to Moon Percival) and barrier autoskill/memoir to survive the opening act is also feasible, though you will need to have some way to heal afterwards. Suiren's counterheal is still useful, though she herself won't survive. Using Andromeda to aggro the opening act is also possible, though she will die to this.
The Baseball Bat Returns
By using a copious amount of bulk US and barrier autos to raise Awakened Mahiru's HP, and having her alone survive (with one US allocated to act, lest you hit the turn limit), you have her bonk Percival for more than twenty turns to come away with a victory.
Another Solo Act!?
Not to be outdone, a Strength Futaba who has enough bulk to survive the opening act (4k barrier memoir +55% HP) works as well. Using her birthday memoir may reduce the US requirements.
Sacrificial 20% Counterheal
With Suiren, the most advisable strategy is loading one strong flower attacker (Nobunaga or Kondo), one tank, and three unit skill sticks that are not flower. Only the tank and the flower attacker will be alive post opening act, and then you can go to town on Percival.
Andromeda
With her eating up the opening act, the other four team slots should be allocated to make a strong flower team that defeats her normally.