TL;DR - Single tap E into Heavy Attack.

Alternate Mods

Nothing. Everything is mandatory.

Riven recommendations

Combination of: Melee Damage, Crit Damage, Combo Efficiency and Initial Combo.

Combat approach

Single tap on E (or whatever you bound melee to) and then Heavy Attack until combo counter is expended.

This is a hybrid build that requires some level of proficiency in execution. Single tap on melee performs two quick shots with [High Noon]'s stance which increases your combo counter more than you can expend with a heavy attack.