Playstyle: Activate Nourish. Use 2 and 3 to place Slash procs on nearby enemies. Use 1 for Absurd Crits. The Crit Chance is Additive to your weapon's crit chance. So, if your weapon has 50% crit chance, and you use his 1 with 100% ability strength, your weapon will have 250% crit chance. That means at 300% ability strength, you get an additive 600% crit chance, which means Guaranteed 6th Tier Crits (Color will stay Red but damage keeps scaling every 100%).

Mods: First up are the Umbral Mods. [Kullervo] has no shields, so we need as much Health and Armor as we can get. The Strength boost is nice as well. Next, [Adaptation] provides us some nice damage reduction.

The true bread and butter for [Kullervo] is [Carnis Carapace]. We will be using Heavy Attack, obviously, so 30% Evasion and 6 seconds of Status Immunity is GODLY. Paired with his Overguard, [Kullervo] isn't going to be paying for his crimes that easily.

Finally, [Primed Flow] lets us hoard more Energy from Nourish. [Primed Continuity] and [Transient Fortitude] boost Strength and Duration so that Nourish, Collective Curse, and crit chance last longer.

Aura: [Steel Charge] is the most obvious pick and allow us to fit in the Umbral Mods. If you Don't mind a second Umbral Forma, you could use Growing Power or Swift Momentum.
If you feel like you want more room, Swap Primed Continuity and Transient Fortitude -> [Streamline] and [Gladiator Resolve].

Arcanes: Molt Augmented for more ability strength, fairly straightforward. Arcane Reaper provides more Armor and heal Rate on melee kill, further improving survivability. This is more reliable than Arcane grace or Arcane [Guardian], as we don't want to NEED to take damage to trigger these buffs.

Subsume: Our melee will most likely be modded for other elemental damage, so Nourish is the best to use here. The Viral Damage and Energy Boost Nourish gives is quite useful. We are healing enough anyways, however with the new update [Kullervo] has much more survivability with his increased Health and Overguard.

Weapon Loadouts
Primary: Daikyu
Secondary: Pyrana Prime
Melee: Syam
The goal of this build is all around synergy. One my my go-to primaries is always the [Daikyu]. Bows are always a ton of fun in this game, and this build uses Gas damage and multishot to take care of groups on enemies. You can also convert the build to being a mostly crit build using cold procs and headshot damage.

The main reason to use the [Daikyu], however, is for the Amalgam Mod. This gives any [Nikana] the ability to have Lifesteal. In [Kullervo]'s case, he does not have access to shield gating, so lifesteal is critically important. 3% Lifesteal does not seem like much, but that will get you back to full health most of the time.

The [Pyrana Prime] is just a solid secondary. Good slash damage takes more advantage of the Carnis mod. Secondary Dexterity gives a bit of a damage boost, and also 7.5s Combo Duration.

Finally, pick your [Nikana] of choice. Here I've chosen the [Syam], it's unique Heavy Attack deals Heat Damage, at the tradeoff of no forced Slash procs. This may help with larger groups, or "missing" from the Teleport of his 1. The shockwave also benefits from the Crit Chance boost from his 1, so it will also do some solid damage. I've got mine modded for Heat/Corrosion to deal with armor. Alternatively, Electric/Gas will use the same mod slots, and serves for more AOE damage.

Really, pick any [Nikana] that you like, just make sure you have the Carnis mod equipped for Heavy Synergy.