Best Team for Fischl in Genshin Impact
Team #1: Eula Superconduct
Xinyan works as a Sub DPS and shielder and provides Physical resistance shred through her shield and her C4, if owned. Xinyan's burst also gets a DPS Bonus in this team. This flexible slot can also host a Crowd Control character, such as Venti.
Team #2: Sukokomon
Sucrose gives free Elemental Mastery to the team and enhances reaction damage, as well as shredding elements with 4pc Viridescent Venerer. Sucrose deals damage as well in this team thanks to the enormous amount of Swirl reactions she triggers.
Team #3: Tazer
Xingqiu works as Beidou and Fischl's Electro-Charged enabler and contributes with some Hydro damage, furthey increased by Sucrose's kit.
Team #4: Klee Overload
Klee's main source of DPS is her ability to trigger the Overload reaction, which does AoE Pyro DMG. This team and Overload in general is recommended only at early stages of the game, as transformative reactions perform better than amplifying ones (melt/vaporize) without proper late-game builds. Klee needs Electro characters to enable Overload and a shield, as she's pretty squishy, downside of being a Catalyst user.
Beidou's Burst's coordinated attacks and Electro application provide huge DPS increase to the team. Beidou however needs another Electro teammate to keep her Burst active for as much time as possible.
Thoma's role is to prevent Klee's interruptions with his shield. Thoma's value here is his being Pyro, which means Pyro resonance and extra ATK for the team, which is very valuable when transformative reactions are used. However Thoma might me a niche character to build and use, so any other shielder does the job too.