TL;DR - Jump in & attack with basic combo.

Stance

[Final Harbinger] slightly edges out [Eleventh Storm].

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 place of [Virulent Scourge] for similar performance.

Riven recommendations

In order of preference; Attack Speed, Critical Damage, Toxin only.

Combat approach

1. Get close.
2. Melee with the neutral combo.
3. Repeat.