Build for my Nikana Guide:

Primed Reach - Increases AOE so you hit further and hit more enemies, which helps build combo count quickly.
Blood Rush - Scales with combo count.
Amalgam Organ Shatter - Increases the damage of critical hits, wind up speed is great for heavy attacks. Can be regular Organ Shatter if you don't care for heavy attacks. (Note: Nikana heavy attacks force Slash proc)
Primed Pressure Point - Increases melee damage, must have for stat-stick weapons.
Berserker - Highest Attack Speed boost, can be Primed Fury instead if you prefer the consistency.
Focus Energy - Electric mod of choice for Corrosive, Melee-Combo Efficiency is useful when you need to Heavy Attack.
Primed Fever Strike - Best elemental mod available to melee, builds into Corrosive.

Final slot is a flex slot ([Gladiator Might], [Sacrificial Steel], [Drifting Contact], Riven)