Fischl can be a powerful DPS unit or a versatile support unit, mostly due to the her ability to summon Oz and enable consistent Elemental Reactions. She also gets a considerable damage increase when you unlock her first constellation, increasing the damage of her every hit when Oz is not on the field.
Whether building Fischl as DPS or support, you should as much as possible invest into her weapon and feather. For DPS this increases the damage output of her Constellation 1 effect alongside her Normal Attack DMG. In support builds, increasing Fischl's ATK increases the passive Lightning Smite's damage for some extra damage on top of your elemental reaction.
When building Fischl as DPS Compound Bow is a great F2P option. Playing Fischl as DPS means you will always be tapping the Normal Attack button, letting easily get max stacks on Compound Bow's passive. Additionally, with Constellation 1 on Fischl, you'll be dealing an extra ATK DMG on top of every Normal Attack. The Gacha equivalent of this weapon would be Rust, which gives a significant DMG boost just with its passive.
If you have them, bows that increase Crit Rate are also great for boosting DPS. Top examples of these are Skyward Harp, Alley Hunter, and Viridescent Hunt, with Viridescent Hunt being the most attainable of the bunch.
Every 4s a character is on the field, their ATK increases by 4% and their CRIT DMG increases by 4%. This effect has a maximum of 5 stacks and will not be reset if the character leaves the field, but will be cleared when the character takes DMG.
Upon hit, Normal and Aimed Shot Attacks have a 50% chance to generate a Cyclone, which will continuously attract surrounding enemies, dealing 40% of ATK as DMG to these enemies every 0.5s for 4s. This effect can only occur once every 14s.
Using Fischl as Main DPS, your Arifacts should focus on Crit and ATK stats. Ideally, substats should also focus on Crit and ATK. Equip a 2-piece Gladiator and 2-piece Bloostained Chivalry mix. If you don't have them yet, Brave Heart, Resolution of Senjourner, and Martial Artist also have similar 2-piece bonuses. Get an ATK% timepiece, Physical DMG% cup, and Crit DMG or Crit Rate headpiece.
Increases Normal Attack and Charged Attack DMG by 15%.
After using Elemental Skill, increases Normal Attack and Charged Attack DMG by 25% for 8s.
Ideally all your Artifacts should have a Crit Rate sub stat so that you can efficiently use a Crit DMG headpiece. If running a Crit Rate Bow, you should have enough Crit Rate to run a Crit DMG headpiece regardless of total Crit Rate substats. Another way you can fully invest into Crit DMG is by equipping a 4-piece Berserker set, at the cost of needed to slightly manage your HP.
Support Fischl is more lenient with her Artifacts because her only role would be applying Electro. But if you really want to maximize DMG, you should look towards her LIghtning Smite passive. Get ATK% on timepiece, cup, and headpiece main stats so Lightning Smite will hurt more. Also ATK and ATK% on substats, no need to give her Crit substats. Artifact sets that give ATK% are prefered such as 2-piece Gladiator, Brave Heart, and Resolution of Senjourner.
If Fischl is the one triggering the reactions, focus more on her Elemental Mastery than ATK. This means an EM timepiece and headpiece with more EM substats. You should also equip 4-piece Thundering Fury for more reaction DMG.
Increases damage caused by Overloaded, Electro-Charged, and Superconduct DMG by 40%. Triggering such effects decreases Elemental Skill CD by 1s. Can only occur once every 0.8s.
For support Fischl your weapon should depend on who is triggering the Elemental Reaction. Fischl's Elemental Skill applies Electro at a very quick rate, so when paired with someone like Bennett who will mostly be the trigger, it might be better to focus more on Fischl's ATK. Prototype Crescent is a decent F2P option for this.
However in cases where Fischl is the reaction trigger, you will need to build a lot of Elemental Mastery to maximize DMG. Characters such as Klee can outpace Oz in applying element. In this case Stringless is definitely the best option.
Increases Elemental Skill and Elemental Burst DMG by 24%.
Main DPS Fischl would function better if the opponent's defense was debuffed by Super Conductor, so Kaeya is a good pair because of his short Elemental Skill cooldown. Additionally it would be good to have Pyro characters on the side to trigger overload and activate the Pyro Elemental Resonance for additional ATK boost. But because you want Fischl to be out most of the time, lingering Pyro applications such as Xiang Ling and Bennett are good teammates.
Support Fischl is usually used in Overload comps alongside Pyro characters. The Pyro characters mentioned above, Xiang Ling and Bennett, are still great teammates just with roles reverse cause now you want to, for example activate Oz then switch into Xiang Ling for DPS. Klee is especially good because she already has great DPS, and Fischl Overload support just adds more damage on top. Since Klee is able to apply Pyro faster than Fischl, Oz will become the reaction trigger. This means you can put the burden of stacking EM on Fischl while Klee focuses on her ATK and Crit stats.
Normal Attack Performs up to 5 consecutive shots with a bow.
Charged Attack Perform a more precise Aimed Shot with increased DMG. While aiming, the dark lightning spirits of Immernachtreich shall heed the call of their Prinzessin and indwell the enchanted arrowhead. When fully indwelt, the Rachsuchtig Blitz shall deal immense Electro DMG.
Plunging Attack Fires off a shower of arrows in mid-air before falling and striking the ground, dealing AoE DMG upon impact.
Summons Oz. The night raven forged of darkness and lightning descends upon the land, dealing Electro DMG in a small AoE. For the ability's duration, Oz will continuously attack nearby enemies with Freikugel. Hold to adjust the location Oz will be summoned to. Press again any time during the ability's duration to once again summon him to Fischl's side.