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.
Observe this incredibly beautiful Yuyuko (which almost makes me regret quitting, then I remember she's 0.2%) here. She gives her party a passive 20% HP Counterheal for two turns. This, of course, nicely mitigates the damage of the opening act for all cloud girls above 20,000 HP, but is much less effective on neutral element girls. The damage is just that high.
This collaboration memoir, which applies a 4000 damage barrier of both types to your team, can provide a slight breakwater against her opening damage. The usage of a Moon Damage -15% from the VS Shop can also be considered.
1) That alone doesn't make it a piece of cake.
The usage of counterheal (practically limited to only Snow White Misora) turns Percival into a team-healing machine, but without that her damage is not insignificant, and will grow over the course of the battle. Some level of stat boosting from unit skills and an array of powerful cloud girls will be required. Without counterheal, Percival's damage is enough to wipe a team with few bulks in a few turns. Empress is invaluable as a support for such teams.
Your creation jewels, please!
Consists of Snow White Misora, one Cloud Midrow besides Venus, and three unit skill sticks.
Snow White should take around 44,000 to 48,000 damage, taking into account criticals, from the opening act, and another up to 7,500 from the first hit without counterheal. With that in mind, being able to take 60,000 damage should be a good baseline for Snow White. Usage of Moon Damage Down memoirs or barrier memoirs will help in reducing this requirement.
Your other selected cloud midrow may have even lower SDEF and HP, which may make her baseline survivability even more difficult to reach. Usage of the Dance of the Gods memoir (5-6k HP bonus) will significantly improve survivability once multiplied through stacked unit skills. The below team can use an offensive memoir on Edward Claudine purely because of how high the unit skills are stacked.