Xilonen's main job is to decrease Elemental Resistances of opponents using her Samplers. This, paired alongside the ability to hold (4), makes her a great asset to many teams.
Xilonen's Burst will heal the active character at intervals if 2 or more of her Samplers had their elemental types changed. Otherwise, she will deal additional instances of Geo DMG.
The resistance shredding from the Samplers occurs when Xilonen is paired with 2+ PHEC (Pyro/Hydro/Electro/Cryo) members. To activate them, press Skill, and continuously Normal Attack until her Nightsoul Points are full. This takes 2 or 3 Normal Attacks to do.
At [C2], Xilonen's Geo Samplers are always active, enabling her to be added in teams with 2 or more Geo members already present.
Regarding Weapon Choices:
Weapon options below do not really affect Xilonen's performance as a Support.
Peak Patrol Song: The buff from this weapon caps out once Xilonen reaches 3,200 DEF.
Freedom-Sworn: This option is competitive with if the team can utilize both of its buffs. To proc it consistently, you either need to Burst every rotation, or add a delay between the initial Skill hit and normal attacking. It is not needed to level this weapon for Xilonen.
Flute of Ezpitzal: Refinements for this weapon are insignificant on Xilonen.
Regarding Artifact Choices:
If you do not need Xilonen's healing ability and are not using , her artifact stats do not matter.
Sets
Scroll of the Hero of Cinder City (4): If no other member of the team can hold it, (4) should be the top priority for Xilonen. The other options are there in the event someone else has occupied.
Main Stats
CRIT Rate: Only recommended if using a Favonius weapon.
Substats
Energy Recharge: Prioritize until requirements are met.
CRIT Rate: Only recommended if using a Favonius weapon.
Regarding Talent Choices:
Burst: If you do not need Xilonen's healing, her Burst can be ignored.