Both Fischl's Skill and Burst summon Oz, therefore it is best to alternate between them to have high uptime.
A big portion of Aggravate Fischl's damage comes from her Ascension 4 passive. An on field unit that frequently triggers Electro related reactions helps maximize her damage.
Regarding Weapon Choices:
Polar Star: Perform a quick Normal Attack and Charged Attack before summoning Oz for more stacks.
The Stringless: At [R5] this weapon performs similar to .
Song of Stillness: Assumes Fischl is healed before summoning Oz.
Alley Hunter: At [R5] this weapon performs similar to .
Regarding Artifact Choices:
Substats
Energy Recharge: Prioritize until requirements are met.
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Indicates important notes at the bottom of the page.