Kaedehara Kazuha's 4th ascension passive provides an elemental DMG bonus buff depending on his Elemental Mastery, putting the main focus on his build on stacking Elemental Mastery while also having enough Energy Recharge to Burst every rotation, as his Burst extends the duration of his passive and deals respectable damage through Swirls.
The element which is absorbed by Kaedehara Kazuha's Burst follows the priority of Pyro > Hydro > Electro > Cryo, and can be absorbed from both the active character and enemies. (i.e. Standing in Bennett's Burst will result in Kaedehara Kazuha's Burst infusing Pyro as Bennett's Burst applies Pyro to the active character).
Regarding Weapon Choices:
Favonius Sword: Try to build some CRIT Rate if using this weapon to trigger the passive consistently.
Xiphos' Moonlight: At [R1], this weapon usually performs worse than due to providing less effective energy to the team. At higher refinements, this weapon can rival in terms of value due to how much Energy Recharge it provides to Kaedehara Kazuha and the team.
Sacrificial Sword: This weapon is mainly used for the Energy Recharge it provides. The Skill reset should not be used unless the extra grouping is needed.
Iron Sting: This weapon can perform better than if you can get enough Energy Recharge from substats.
Toukabou Shigure: This weapon can perform better than if you can get enough Energy Recharge from substats.
Regarding Artifact Choices:
Sets
Thundering Fury (4): This set is only used for a few specific teams, in which Kaedehara Kazuha is used on field to deal damage through Aggravate Swirl hits.
Substats
Energy Recharge: Prioritize until requirements are met.
CRIT Rate: Only recommended if using a Favonius weapon.
Regarding Talent Choices:
Kaedehara Kazuha generally does not need his talents leveled on an Elemental Mastery build.
Normal Attack: Upgrading Kaedehara Kazuha's Normal Attack talent increases the damage of his plunge attack after using Skill.