Naramon focus school + Energy Nexus Warframe mod.

Alternatively use Zenurik focus school and put Primary Dexterity and Secondary Dexterity on your primer or throwaway guns.

Probably do the first one since I only use hate on kullervo. Thanks though

Thats why the Dexterity arcanes are good for that scenario though, as they passively increase your combo duration by 7.5s each (at max rank only).

Kullervo's signature shotgun, the Rauta, also passively gives Kullervo 7 seconds of combo duration for his melee.

Makes for +22 seconds on weapons you don't even need to touch inside a mission.

I generally prefer using Naramon and bringing alternative energy sources though.

Efficiency and energy. Nexus

Another alternate is to run a zaw with exodia brave and nourish over his fourth. Preferably the balla. If you ever decide to switch it up from hate.