TL;DR - Charged attack on ground

Stance

Doesn't matter. Both have forced slash procs on charged attack.

Alternate Mods

Swap [Primed Fury] for [Quickening] for virtually no loss but you'll still need that many forma unless you're willing to spend an Umbral Forma for this.

Riven recommendations

Combination of: Critical Chance, Melee Damage, Crit Damage, Initial Combo.

Combat approach

Remember to move around while winding up your charge attacks. Otherwise nothing much to say except watch the video.