Riven-free, Anti-Eidolon, 4 Forma [Rubico]
[Rubico Prime] is currently the best sniper rifle in the game, boasting incredible Critical stats and Damage while still having good firing rate, status and more than 2 shots in the magazine.
Split Chamber: Staple in any primary, no explanation needed, huge damage boost.
Serration: Also a must-include mod, with one exception - [Chroma]. Otherwise not using this mod is like cutting your arm off. No explanation needed.
[Point Strike] + [Vital Sense]: [Rubico Prime] is a Crit weapon, and these are crit mods. And since we won't be doing much headshotting and killing anything other than eidolons (ideally) with this, we don't have other alternatives.
[Stormbringer] + [Hellfire]: Elemental mods, The two together give you a x2.8 damage multiplier (essentially) and make that damage Radiation. Radiation has a huge damage multiplier against alloy armor. Eidolons have Alloy armor. 2+2=4 and the Eidolon is dead.
[Heavy Caliber]: Tons of Damage, This is basically an extra [Serration] with a downside, mitigated by [Rubico] being a Sniper rifle.
[Vigilante Armaments]: While it stacks additively with [Split Chamber], this is still a good mod, as it increases your damage multiplicatively with [Serration] and elemental mods. It's not a huge boost, but it does end up being a chunk. Best 8th mod available IMO.
[Serration]: If you're using [Chroma] you may want to consider replacing this with an elemental mod, due to [Serration] stacking additively with Vex Armor, [Primed Cryo Rounds] or [Wildfire] would be my recommendations
[Heavy Caliber]: If you're getting rid of [Serration] and still have this mod in, just use [Serration] instead of this. Otherwise, consider replacing this with more elemental damage mods like [Primed Cryo Rounds], [Wildfire] or [High Voltage].
[Vigilante Armaments]: If you're open to changing your playstyle up... A LOT, consider replacing this with [Proton Jet]. I know, sounds janky AF, but it CAN work.
Personally I chose [Twitch] Because when hunting eidolons I prefer to play as [Trinity] and I take shooting down the acid orb things as my responsibility, [Twitch] helps me to switch to my secondary and take down the orbs faster.
Another good choice is [Stabilizer] as it can improve your practical DPS by making you feel more comfortable at shooting faster and makes recoil control easier.
On the Forma/mod rank note: I wouldn't bother formaing for such a minor upgrade, thus my [Twitch] is Rank 1, but if you want to use [Stabilizer] or don't have anything better to do, you won't regret putting another forma in.
Thank you for reading my guide and I hope you learned something from it! Check out other guides I've made on this website on the right hand side of the page or by clicking on my name at the top of the webpage. (When I've made more that is) (: