Well, this is a funny shotgun with equally funny numbers.

I tried many things and found out that Cold + Toxin (innate Viral) on the Riven may be the most optimized choice.

• Theoretical stats (from IG values) :

(Arcane Tempo activated, no additional bonus from [Galvanized Hell])

• Practical testing (Proc-Per-Magazine) :

These are extremely close to the theoretical values.

• Practical testing with [Hunter Munitions] (> 6 seconds constant fire for reaching the maximum of slash stacks) :

Once again, numbers are pretty close.

About this build :

• Innate Viral on the Riven helps getting this build right with very little concessions.
• It takes full advantage from 3 mechanics at once : heat procs (armor stripping with DoT and panick effect), slash procs (bleed effect) and viral procs (damage bonus applicable to direct damages and both heat and slash procs).
• Enemies are also weakened by the puncture effect, up to 75% damage reduction.
• Using a Primed faction mod and Primed Point Blank will maximize the damage output due to the bleed effect being very sensitive to both mods.
• From the start (without any buff from the Primary Arcane nor Galvanized mods) it can melt a lvl 135 Eximus Nox without shooting in the weak-point.
• Once Galvanized mods and Primary Arcane are activated, damages drastically ramp up.

Notes :

• Use the correct faction mod.
• I recommend using Arcane Tempo.
• Arcane Primary Merciless is the best choice and easy to maintain.
• Primed Ammo Mutation is mandatory.