Recommended Grip: Tremor(higher dmg) or Palmaris(lower dmg but better range).
If plan on using [Sinister Reach] anyway, you can go for Tremor, tho the dmg difference is barely noticeable for me, but in the end, this is purely preferance based.

Recommended Loader: Killstream(better reload speed) or Splat (better [Mag] size).
If you use Primary Merciless, you may want to use Splat.

This build is to be used together with the Warframe Arcane "Theorem Demulcent".
Theorem Demulcent increases(on max rank) you weapon dmg by 12% with the respective Element of your chosen Residual Arcane(in this case Malodor).
This will cause the innate Toxin and the Cold Dmg from Theorem Demulcent to form Viral (the downside of this is that Viral will be VERY dominant as Status, and your slashes will only proc reliable from HM).

Here I am using Primary Merciless, but you can also use Primary Deadhead, as you WILL get enough Headshots to stack it, yet I chose Merciless for Reload Speed.
We can use a Residual Arcanes as we wont run out of ammo (at least I didn't encounter a situation) thanks to [Vigilante Supplies].
The exilus slot can be forma'ed to Naramon without worries, as the all Exilus you'd want to use are Naramon (Vigilant Supplies, Ammo Drum, Sinister Reach, Twitch).

This is a forma-heavy build, requiring 7 freaking forma, though we're using 2 Galvanized Mods, a Riven and Serration, so it's somewhat to be expected.
There are 2 optional slots(/flex slots(however you wanna call em)). Being [Serration] and [Hammer Shot].

[Serration] is in here by default for "lazy play" (if you don't want to use Faction Mods), but I'd recommend either [Serration], [Galvanized Scope], a Faction Mod or any Fire Rate Mod for this Slot.
[Hammer Shot] is completely optional and can be exchanged with anything you want, though here again I'd recommend [Hammer Shot] or Vigilant Armaments.
If you want the innate Viral, you can also exchange [Serration] or [Hammer Shot] with a Cold mod, be it Primed Cryo Rounds or Rime Rounds.

In best case you'd run a Helios with a 4x Vigilant Setup.

Your optimal Riven would be:
+CC
+CD
-Projectile Speed/Zoom

or

+CC
+CD
+Multishot
-Projectile Speed/Zoom

I personally am using this setup for almost anything, including Lichs, you'll be good to go, but I do NOT recommend this for Disruption, go for a [Phantasma] or such.