[Laetum] is a refreshing antidote to the mindless AoE boom and zoom meta. I would argue that as of Update 31.5 it is the strongest gun in the game (non-arch).

It allows build diversity through compelling perk choices, including this incredibly strong non-crit build thanks to the amazing Evolution V perk.

With this build I recommend the following Evolution perk choices:

Evolution II - Perk 2: Rapid Wrath: +20% Fire Rate.

Evolution III - Perk 1: Lethal Rearmament: On Headshot: +30% Reload Speed for 12 seconds. Stacks up to 3 times.

Evolution IV - Perk 3: Elemental Excess: +20% Status Chance. -10% Critical Chance.

Evolution V - Perk 1: Devouring Attrition: 50% chance to deal 2000% damage on non-critical hits.

Because we are building around Devouring Attrition, we actually want to reduce crit chance as much as possible. That's why we choose Elemental Excess, which applies as a flat increase/decrease after everything else, i.e., crit chance is 12% and status/projectile is 86%.

This is also one of the rare cases where the Deadhead arcane is better than Merciless. We'll be hitting plenty of headshots both in regular fire and Incarnon mode, and -50% recoil makes it easier to stack those headshots -- which also get the nice 30% multiplier increase. And the Lethal Rearmament perk makes Merciless's reload buff superfluous.

As for Helminth abilities to pair with [Laetum], I strongly recommend [Xata]'s Whisper, as Devouring Attrition double dips on the void piece and makes it absolutely insane.

If you are not a fan of faction mods, [Hemorrhage] is a great option for that slot because of the constant procs in Incarnon mode, and will fit without additional forma.

As for warframe arcanes, I am a fan of using Arcane Velocity with this build to make Laety really go brrrrrrrrrrrrrrrrrrrrrr