|Cordelia has respectable attack and speed, allowing her to function similarily to other fast horse lances. However, instead of opting for Shell Lance+, we use Firesweep Lance+ to more safely engage on enemies without being countered.
The choice of ATK+ or SPD+ mostly comes down to buffs available. With Hone Cavalry for example, Peri benefits more from ATK+. Without it, she more consistently doubles enemies with SPD+, making it the superior choice.
Peri's assist skill is flexible and should be selected based on your team's needs.
Luna is picked as a solid option to add additional damage to Peri's second engagement. As she should not be counterattacked, it will activate on her second attack, should her first not KO the opponent. You can also run Moonbow alongside Quickened Pulse to maximize damage on the first engagement, making it an effective option.
Life and Death 3 is picked in order to buff Peri's damage output and ensure that she has high enough speed to double most of her opponents. Her high speed also ensures that she will almost never be doubled by any unit without a brave weapon, which is essential to survive and defeat any enemy that is able to return damage to her. The skill also benefits from Firesweep L+, as it keeps Peri from taking damage when initiating combat. Swift Sparrow 2 deserves mention for keeping Peri slightly safer on the enemy turn, although it makes her slightly more likely to doubled by certain glass cannon units, such as Nino. It is also rather rare, so Life and Death 3 is preferred.
Her B skill has a couple options, mostly coming down to preference. Drag Back allows you to safely draw a dangerous target into range for an ally to defeat, while Seal Attack will allow Peri to survive a few more key encounters by herself--namely Fury 4 Nino: Pious Mage or Spring Camilla without buffs. It also allows her to safely debuff powerful enemies for her teammates to engage.
Her default C skill, Threaten Def, functions perfectly fine and does not need to be changed.
While Peri faces stiff competition as the role of a blue cavalry unit, she stands out as the most viable of them capable of doubling enemies safely for physical damage, giving her a slight niche. She is also more common of a unit than other top blue cavalry units.