This is the absolute min-max build you can get for Eidolons with this weapon.
It assumes you fully strip the Eidolon's Armor to get the bonus damage from Electricity.
Progenitor Bonus must be Radiation to receive the +25% Damage Modifier against Robotic Health (the health type Eidolons use when their Armor is stripped.)

Assuming you're using Void Strike and Roar/Eclipse, your damage should be more than enough to break the Synovia with 1 shot.
During its falling animation and Energy Spike, strip its armor.
You'll set yourself up to deal maximum damage without relying on Void Strike.

Best Possible Riven: CC/CD/D: 64.2% MINIMUM

Riven Hierarchy:
Damage: 27.7%
Critical Damage: 24.0%
Critical Chance: 19.0%
Electricity: 16.0%
Multishot: 16.0%