[Rubico Prime] build made specifically for Eidolons.
Mods explanation:
For eidolons, since you can't apply status effects on them, we are looking for pure crit chance and crit damage. We are modding for radiation since they are alloy armored aka we will be gaining a +75% dmg increase to them. Make sure when you're armor stripping them to reduce their EHP to not full strip them otherwise their health goes robotic and your modding for radiation becomes a +25% dmg increase instead of a +75%.
Exilus slot: Personal QOL, you can fit in either Ambush Optics like I did for lower zoom, or Twitch for holster speed (tho this becomes redundant if you're running a holster speed aura or if you're running Vigorous Swap on your frame), or you could run Ammo Drum for the ammo maximum since you'll be firing a lot of shots on the eidolon during it's invulnerability just to stack up the combo counter of [Rubico]. You could also run Vigilante Supplies purely for that crit increase in order to get a higher crit damage multiplier for more damage, altho this one is the least useful one out of the 3 since it's low % and rng based.
Serration is needed but can be replaced with Amalgam Serration for the extra movement speed (this will not affect your archwing flight speed tho, unless you're [Titania] in Razorwing, in which case I don't know what you're doing with her in Eidolon hunts). Please do not run Heavy Caliber as it gets it's damage value reduced when ran together with Serration and it also lowers your accuracy meaning that your shots or multishots of your weapon might spread and miss their target, aka the Eidolon synovia (the weak spot). They are quite big but it does in rare cases sometime happen.
Split Chamber and Vigilante Armaments are your multishot mods. They push you to 2.5 multishot and they're both needed since without vigi armaments, you'd be at 1.9 multishot.
Critical Delay and Vital Sense are your bread and butter cc+cd mods. Critical Delay is much better than Point Strike since it gives 50% extra cc and it pushes us past 100% cc.
Vile Acceleration is crucial since it boosts our fire rate which helps us stack up our combo counter faster as well as shoot our 5 magazine shots on the Eidolon synovia faster when it's vulnerable. Remember, fire rate is very much appreciated and is the main reason why [Wisp] replaced [Trinity] as the lure nanny in 4man hunting squads. (before [Trinity] mains come and get mad because of this comment, I'm sorry I play [Trinity] as well, but it's true, your only purpose is Blessings heal and it's overkill for Eidolon hunting these days)
Hellfire and Stormbringer are our standard heat+elec=radiation mods. Do not run 60/60 mods as they provide less raw damage and the status chance is a dead stat. If you're running [Volt] or have [Volt]'s shock subsumed from helminth, you can run his augment mod Shock Trooper which will coat your weapon with electric damage that gets combined with the mods you have in your weapon, which means that you'd be able to drop Stormbringer completely for some other mods like a Riven or Hammer Shot for the extra cd, Argon Scope for the extra cc (on headshot tho, so you'll have to shoot the Eidolons head to proc the buff before his synovias become vulnerable, this cc buff is also diluted with Critical Delay and it won't push you past 200% cc unless you're using external cc buffs like Arcane Avenger and Adarza Kavat's Cat's Eye since these give you flat +45% and +60% cc increase) or Primed Chamber (hehe have fun getting that one). I'd also like to mention that you could run [Oberon]'s Smite helminth subsume and his augment Smite Infusion which coats your weapons with radiation allowing you to drop both elemental mods which would in turn open up even more mod slots for the previous mods I've mentioned.
Primary Merciless is useful since at rank5 it reduces our reload speed from 2.0 to 1.5 and gives us more ammo to work with. We do not care about the damage per kill increase since there will be nothing to kill for us, our main target is the Eidolon. Feel free to use Primary Dexterity for the +60% holster speed at rank5 (tho this becomes redundant because of the points I mentioned in the beginning of the guide) or Primary Deadhead for the weapon recoil.
Honorable mod mention would be Hammer Shot since even tho it gives status chance which is completely useless for Eidolons, the critical damage it gives is noticeable because of [Volt]'s shield giving 200% cd which is calculated at the end of the calculation aka it doubles your current critical damage. So without [Hammer Shot] with [Volt]'s shield + [Rubico]'s +50% cd our critical damage would be 13.2% but with [Hammer Shot] it would be 16.8%.
Lastly I'd like to mention that a Riven mod absolutely makes this weapon broken since any extra addition of critical damage is a blessing because of [Volt]'s shield. The Riven dispositions of weapons are always changing but as of currently writing this guide, [Rubico Prime]'s riv dispo is 0.6% which is extremely low but It's still useable. If you're wondering which stats your [Rubico] riven should have, look for positive stats in this priority:
1) Critical Damage(because of [Volt]'s shield interaction with the calculation)>
2) Critical Chance(gets diminished by Critical Delay but helps push for red crits with the external crit helpers that I mentioned above in the guide)>
3) Heat(best raw damage increase, don't go for Electricity since that will be hard diminished from stacking 6 [Volt] shields that give +50% additive electric damage per shield)>
4) Multishot(the only noticeable stat change would be you reaching 3.0+ multishot (aka your riven needs to have minimum 50% multishot))>
5) Damage(close to useless, gets diminished by Serration)>
6) anything else is useless
For negative stats look for something like -Zoom, helps you aim, can drop Ambush Optics then and run some other QOL mod.
I hope you enjoyed the read, good luck and have fun in your hunts.
