Mika is a Physical damage support who offers Physical DMG% through Detector stacks, and Physical CRIT DMG at C6. He is additionally able to heal and provide some Attack Speed.
Do note that Mika's buffs apply to your on-field character and do not linger, this is something to keep in mind especially when playing him with Eula, as you will lose his buffs if you detonate her Lightfall Sword early.
Regarding Weapon Choices:
Symphonist of Scents: Keep in mind that without his C4, it can be very difficult to meet his Energy Recharge needs to Burst every rotation and make full use of this weapon.
Favonius Lance: This is often Mika's best in slot option, as both an Energy Recharge weapon and one with a useful passive which generates particles on Crit. Weapons which provide Energy Recharge are prioritized to ensure good Burst uptime.
Engulfing Lightning: Weapons which provide Energy Recharge are prioritized to ensure good Burst uptime.
Prototype Starglitter: Weapons which provide Energy Recharge are prioritized to ensure good Burst uptime.
"The Catch": Weapons which provide Energy Recharge are prioritized to ensure good Burst uptime.
Regarding Artifact Choices:
Sets
Ocean-Hued Clam (4): A good option if another party member is using (4).
Scroll of the Hero of Cinder City (4): Not very helpful for teams that focus on Physical DMG.
Main Stats
CRIT Rate: Only recommended if using a Favonius weapon.
Substats
CRIT Rate: Only recommended if using a Favonius weapon.
Regarding Talent Choices:
Burst: Prioritize it if you value additional healing.