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

Stance

[Decisive Judgement] outperforms both [Blind Justice] and [Tranquil Cleave] in all areas.

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