TL;DR - Jump in, slide attack, attack with basic combo.

Stance

[Gemini Cross] and [Sovereign Outcast] are pretty similar in terms of DPS output, proc quality & quantity, and multihit & multipliers.

Alternate mods:

[Primed Fury] can be dropped for [Quickening] with about 20% DPS loss or [Primed Reach] for more range. [Organ Shatter] can be slotted in but with less than 120% additional damage it is slightly undervalued compared to dropping Viral or [Primed Pressure Point] since Viral will make up for 100% of the damage loss and the additional proc for CO will easily overcome the remaining difference.

Riven recommendations:

In order of preference; Attack Speed, Viral (having both Toxin and Cold), Critical Damage, Toxin only.

Combat approach:

1. Open with a slide attack. [Vacuum] range is 14m so you can be a bit free with the positioning.
2. Melee with the neutral combo.
3. Wait for the Vacuum cooldown to end. You will otherwise waste time by pushing enemies away and/or put it on cooldown unnecessarily.
4. Repeat.