Aya

Composed

Middle row

Magic damage

Witches

DPS

Ice Butterfly

Sends an ice butterfly to the enemy. It flies a a set distance and returns. In light, the butterfly collides with enemies, dealing Magical Damage multiple times and reducing their Skill Damage. If the butterfly collides with an enemy, you recover you're SP.
SP Recovery effect activates only once on the same target.
■ Magical DMG Per Hit: 425%
■ Skill DMG Reduction: 50%
■ Skill DMG Reduction Duration: 6s
■ SP Recovery: 15%

Frost Flower in Bloom

28 sec Cooldown

A frost flower blooms on the enemies, dealing AoE Magical Damage and applying Freeze. Blooms a new frost flower on a random hit enemy, dealing AoE Magical Damage to surrounding ones. The frost flowers bloom only once on the same target.
Freeze: Applies damage every second and increases Outgoing Damage from Composed Apostles.
■ Magical DMG: 415x%
■ Max Repetition Count: 5 time(s)
■ Incoming DMG Increase from Composed Apostles when in Freeze: 8%
■ Freeze Duration: 10s

Passive

Reduces Incoming Critical Damage.
Increases all allied Composed Personality Apostles' attack.
(This effect activates even if Aya is not on the field).
■ Incoming CRIT DMG Reduction: 42%
■ ATK Increase: 23%

Normal ATK

Swings her Snowflake Blade, dealing Magical Damage 2 times to the enemy.
Total Magical DMG: 100%
Enhance:
At a set chance, swings her Snowflake Blade, dealing AoE Magical Damage 2 times to enemies. The second attack reduces the enemy's Attack Speed.
■ Total Magical DMG: 190%
■ ATK SPD Reduction: 20%
■ ATK SPD Reduction Duration: 4s

Skill priority

You will level up most skills regardless how useful they are for the combat power padding in endgame. As a general rule of thumb:

Units you are not sure you will use long term: Skills to level 4. If they're passives that benefit the team, can take that to 7.
Strong units but not your main personality: Skills to level 7 for raids and elemental content. Not worth going higher if they are not your main team.
Strong units on your main personality (and waifus): Max out skills. You can start with 7/7/7 and slowly work towards 12/12/12 when you can sweep stage 10.

Recommended artifacts

Artifacts and runes will always change depending on the stage and content you are currently facing, this is just a couple of examples on what items work for this Apostle.

Legendary