My Akarius for casual, chilling gameplay which does not require much effort but giving out great efficiency

Primed Sure Footed - it's necessary for this build.
There are also other ways to avoid knockdown: Rhino - Iron Skin, Revenant - Mesmer Skin, Gauss - Kinetic Plating, Nezha - Warding Halo, Titania - Spellbind, Nidus - ParasiticLink, Oberon - Hallowed Ground, Trinity - Link, Yareli - Merulina, Atlas passive ability

Basically, this build is based on arcane procs: Arcane Awakening, Arcane Pistoleer (Headshots kills procs still work)
*Arcane Precision - doesn't work anymore.
For Helminth abilitiy - Energized Munitions

After Patch 32.0 surley this build gets nerfed, but I figured out working and quite comfortable build in which I replaced Arcane Pistoleer to Arcane Awakening, and replaced Pistol Ammo Mutation to Trick Mag
Also i recomend to use Carrierl with Ammo Case

About build:
Instead of a riven mod, you can use Hornet Strike, Magnum Forcel, Primed Expel Grineer, Primed Expel Infested, Primed Expel Corrupted, Primed Expel Corpus
Magnum Forcel instead of a riven mod can increase ur headshot arcane procs, simply because it reduces accuracy, which can cause projectiles to hit the head.
Primed Fulmination - will increase AoE of explosions, that can deal damage through surfaces, objects.
Primed Quickdraw and Secondary Merciless - neutralizes weakness of Acarius in terms of reloading. Remember that when sprinting, you reload Acarius faster.

Because you kill evreything with Acarius, you can make Warframe mobility based build for faster movement.
Warframe build

Also, you can bind the shoot button to the mouse wheel so that you don’t get tired of constantly clicking on M1
With this build you can do ESO, Arbitrations with high efficiency.