[Rubico Prime] is better until you get a Riven, but I think the [Lanka] outstrips it once you build it out. [Rubico] is certainly smoother to use, at least, because you don't have to charge each shot.

Galvanized Mods: No reliable way to get kills in the Eidolon fight. Yes, they would be better if you could keep the stacks up, but since you can't, [Split Chamber] > [Galvanized Chamber].

Arcanes: Same as Galvanized mods. No way to keep the stacks, so it's difficult to even keep up. I leave it off the build here so it doesn't distort the numbers, but the numbers don't provide a good representation anyways. You might as well put it in, because you're just leaving extra damage on the table just in edge cases. Arcane Deadhead is what I have in mine.

Damage/Armor: Eidolons have Alloy Armor (75% weak to Rad, 25% weak to cold), Robotic Health (25% weak to Rad), and Shield (shot with amp, so don't bother building for this). Robotic Health will take more damage from electricity, but there's no feasible way to get the pure electricity up that high, and Rad is a HUGE damage multi against Alloy Armor. You want to keep the Alloy Armor on so that you get that huge damage increase, but reducing it is a good idea, because it should be a 33%+ damage reduction.

I will have a video of me soloing an eidolon to demonstrate the damage. I'll probably compile just the highlights in the video next, the following link is the full fight, one take, no editing. Just dumped a raw OBS capture into youtube