Best used as a heavy attack spam weapon with 5x initial combo.
After further testing, gas seems to hold a better sustained dps if we're only accounting for heavy spam. (Use Roar/Faction mod to maximize damage).
If trying to optimize with blast, your dps will come from light attack spam and occasional tennokai heavies (Drop galv reflex, use [Xata]'s whisper and faction).
Because blast deals more damage based on the # of procs, it is ideal to use a weapon with lots of multishot or attacks per second. [Syam] heavy spam is not that. [Syam]'s light spam however? Yes.
Gas does benefit from gas stacks, but gas stacks only add radius or another gas instance, rather than buffing the current one. Gas also deals damage the instance it is applied. This makes gas a heavy spamming monster and great at holding positions.

I USE ARCANE FURY FOR BASE DAMAGE ON THIS WEAPON. IF YOU DON"T HAVE IT, USE PPP/KILLING BLOW OVER BERSERKER"S FURY OR GALV ELEMENTALIST.

• Dropping base damage and using arcane [Fury] is more effective than dropping attack speed and use arcane strike.

This build is for hybrid gameplay, weaving normal attacks for tankier enemies and heavies for crowd clearing.
Alternatively, if you'd rather spam heavy attack, you can drop PPP and [Galvanized Elementalist] and replace with [Killing Blow] and [Melee Elementalist] for a cheaper version with hella fast heavy startup.

Inspired by: Ninjase's Syam Build but without crit chance and wind up speed, due to Kullervo innately solving that for us.

Arcanes:
Melee Influence (Maybe run a grouping companion/weapon).
Melee Exposure for raw damage.
HM: Melee Duplicate can be very fun, however because this is a build for [Kullervo], melee duplicate has no place as Wrathful Advance pushes crits past melee duplicate's condition.

Mods:
Condition Overload and PPP/[Killing Blow]: mandatory damage, both mods scale multiplicatively together on heavy attack. Because of this, both PPP and [Killing Blow] are both good options, with killing blow's heavy wind up speed making it slightly more favorable (not that kullervo needs more windup speed w/ his passive). NOTE: because basic attacks are additive w/ CO, PPP isn't as effective as it's listed value, as it has diminishing returns with CO. Can also prefer to run attack speed over PPP/KB and use Arcane Fury to supplement.
Galvanized Elementalist/Melee Elementalist: Status damage, with status chance or heavy windup speed.
Galvanized Steel: Crit Damage
Vicious Frost/Virulent Scourge and Volcanic Edge: Blast and gas DoT damage cannot be increased using modded heat and cold/toxin mods. That means that there is no need to run the 90s mods and waste mod capacity.
[Galvanized Reflex]: I find maintaining combo hard with a combo time of 5 secs, and with reflex you can use heavy attacks to clear crowds without needing to proc tennokai beforehand, making this effective at all ranges (besides vertical targets lol).
[Voltaic Strike]: Proc Influence. Not using [Focus Radon] as with Kullervo, his passive already covers a lot of the heavy attack efficiency (Galv Reflex takes it to the cap of 90% efficiency).

• NOTE: You can technically run melee influence modding only for electricity, however that requires you to basic attack to proc influence before firing off heavy attacks (which will only spread radiation due to the heavy attack's innate heat.) Its much more recommended to mod for gas/blast to combine with that innate heat and allow even heavies to proc influence and spread an actual damaging DoT.

Damage Calculation:
Scenario 1: 5 enemies, not grouped, with blast procs on 1 enemy, assuming the blast damage on detonation deals 10 dmg.
The one enemy gets dealt 10 damage.
= (Blast procs damage on single enemy)

Scenario 2: Same as scenario 1, but grouped and in range.
On the initial detonation of the blast procs, 10 dmg is dealt to the 1, which gets dealt to the other 4 due to blast's AoE detonation dmg, totaling 50 dmg.
= (Blast procs damage on single enemy) x (# of enemies grouped)

Scenario 3: Same as scenario 2, but now with collective curse applied to all enemies.
When that 50 damage is dealt, it is also transferred to the other enemies due to collective curse, quintupling and dealing 250 dmg.
= (Blast procs damage on single enemy) x (# of enemies grouped) x (# of cursed enemies)

Scenario 4: Same as scenario 3, but now we have melee influence active and all enemies are in range.
When we mod with melee influence, the blast procs applied to the first enemy would also spread to the others, essentially replicating scenario 3 for each enemy, a total of 5 times. The 250 dmg will be quintupled again and turned into 1250.
= (Blast procs damage on single enemy) x (# of enemies grouped) x (# of cursed enemies) x (# of enemies influenced)

Scenario 5: Same as scenario 4, but now we're running [Syam] and use it's heavy attack waves with infinite punch through to apply blast and proc influence on all enemies.
Under the assumption that the interactions listed in scenario 4 perfectly replicate for the new enemies, the 1250 dmg will be quintupled once again and turned into 6250 damage.
= (Blast procs damage on single enemy) x (# of enemies grouped) x (# of cursed enemies) x (# of enemies influenced) x (# of enemies hit)

These 5 scenarios show that in an ideal scenario, with grouping + collective curse + influence + punch through, the blast procs can have a quartic relationship. If you were to remove any layer of that complexity, you lower the relationship to a lower power.

This yields the ideal final blast procs damage equation to be:
(Blast procs damage on single enemy) x (# of enemies)^(complexity), with complexity being the number of damage layering you apply (e.g. grouping, curse, influence, punch through).

• NOTE: This is heavily simplified, as some scenarios you won't curse all enemies, or not all enemies will be close enough to be dealt the blast AoE dmg, or you won't proc influence, or you won't hit all of the enemies.

Flexible Slots:
Weeping Wounds - More status = more gas/blast procs. Results in more status procs thatn expected, as influence will spread that increased number of procs. Not too recommended for gas, as all procs leading up to 10 only increase the cloud DoT range, not damage.
Berserker's Fury - Attack speed helps with basic attack spamming and speeding up heavy animation recovery (Melee Elementalist and Killing Blow only speed up windup, not recovery/endlag).
Faction Mod - Double dips on blast/gas, hard to replace any of the above mods, best would be reflex.