I use Shotgun Ammo Mutation because Ferlax is ammo-hungry due to the high fire rate and low ammo magazine. I am using Corrosive Cold to combat all armor types and shields, but Radiation Cold for Archon Hunts, and Viral Electric for 100% Amor Strippers are possible too. I built for raw damage over critical damage due to my evolution choices. Overframe doesn't take into consideration my evolutions, so it undervalues the DPS by Millions less which is entirely incorrect. In the end, this is a very powerful Steel Path Endurance shotgun and my favorite primary weapon of all time. It's a heavy 8 forma investment but if you follow this build, then you won't need any other weapon. Trust me, bro :D

Evolutions:
1: There is no choice, there is only Incarnon mode which I feel is a bit lacking compared to the normal mode.
2: Kinetic Baffle for -50% Recoil. Alternatively, you could use the Primary Deadhead for -50% Recoil and the 2nd Evolution for Projectile Speed.
3: Mounting Momentum: Reload increases Fire Rate by +10% per shell. Resets on reload. This gives a 130+ Fire rate with Primed Ammo Stock. I am using Primed Tactical Pump and Merciless to reduce the long 6.5-second reload speed to 2.8 seconds.
4: Wracking Wrath: + 20% Status Chance and - 10% Critical Chance. Both buff and penalty are perfect on this build, especially when paired with the Tier 5 Evolution.
5: Devastating Attrition: 50% Chance of +2000% Damage on non-critical hits. This is multiplicative damage to base damage bonuses like Primed Point Blank and 165/90 mods. It is also why I went pure elemental damage mods instead of 60/60 mods. I have seen millions of damage on standard hits, this weapon kills everything on sight with one hit.

Flexible Mods: For Archon Hunts, use an Invisibility Frame like [Ivara], use a [Silent Battery], and change [Contagious Spread] for [Incendiary Coat]. If you have a 100% Armor Strip, you can then replace the positions of the [Charged Shell] and [Chilling Grasp] to make Viral Electricity instead of Corrosive Cold. Also, you can remove [Primed Point Blank] for Faction Smite mods for even more damage. I suggest using [Mag] Magnetize or [Xata]'s Whisper to further enhance the damage through its infinite punchthrough property. If you are missing the Primed mods then use their non-Prime variants. However, don't use [Vigilante Supplies], Arcane Avenger, [Biting Frost], or anything that gives a critical chance as it is counterproductive to Devasting Attrition.

Disclaimer: This is a primary mode build not an Incarnon mode build. Incarnon mode is still strong and it even has Radiation on top, but I prefer to play with the normal shotgun mode more. I know not everyone has it, but I actually use Shotgun Vendetta over Primary Merciless, it reduces my reload speed to 2.36 which feels more comfortable to play compared to the 2.8-second reload speed shown above. Get your Drifter Instrtics Opportunity to rank 9, farm Pathos Clamps and purchase Shotgun Vendetta since it is the best arcane for [Felarx] in my opinion. In my honest opinion, [Primed Tactical Pump] and Shotgun Vendetta together are faster than the 3rd evolution Auto Reloader because you waste time swapping weapons to holster Ferlax. Additionally, it gives back the opportunity to use Mounting Momentum for the 130% Fire Rate. The 130% Fire Rate is practically a Primed Shotgun Spazz, making it very good for us. Regardless, even if you don't have Shotgun Vendetta, using [Primed Tactical Pump] and Primary Merciless together is quite good anyway.

Enjoy :3

Here is my Phenmor build, its just as good and requires less forma and primed mods to be as effective. https://overframe.gg/build/527236/