Corrosive/Cold/Radiation Ambassador setup. Would it potentially be more efficient if it was Viral/Slash HM or something? Probably. Do I just not want to run yet another Viral Slash HM build? Absolutely.

This setup, tested on Taveuni SP, nets around a 60kps with no buffs and no companion, capping out at ~300 kills at the end of a 5 minute mission.
Using my [Champion's Blessing] [Trinity] and a Nautilus using a Viral Helstrum, that rose to about 100kps.
The riven is technically optional but really helps (it's about 1/3 of the DPS difference). If you need to it can be replaced with a [Stormbringer] or something and then replace [Radiated Reload] with [Serration] (Or I guess [Galvanized Aptitude] if you don't really plan on using the charge shot). It's not a god roll or anything but it helps a lot.

[Galvanized Scope] is being used because the weapon is pinpoint accurate and even when using the AoE mode the beam itself can trigger headshots. If you don't like aiming, replace it with [Critical Delay].

[Radiated Reload] actually adds a lot of DPS here because the [Ambassador] has a really dire reload speed, which affects the swap time between the two modes and heavily affects your DPS in the Charged mode as you get two shots per mag. If you want to sacrifice a little DPS for more reliable Corrosive/Cold procs you can replace it with a Fast Hands/[Primed Fast Hands].

The arcane kinda depends on which mode you prefer using. If you might want to switch modes and use the Charged shots at all, I recommend Merciless because it will improve the AoE splash more alongside the aforementioned reload speed. If you just plan on using this as a fancy machine gun, go with Deadhead for the headshot multiplier.