Evo II: Final Fusillade (Increase Damage by +51. +3 Multishot on last shot in magazine.) We take this because the other perk (Plentiful Mayhem) is absolute dogshit and actively makes the weapon worse.
Evo III: User preference. I prefer +9 magazine size as it allows you to freely spam shots to get back to incarnon mode quickly, but +50% projectile velocity is good too. The other one seems pretty meh, ideally you never even reload with this weapon.
Evo IV: Survivor's Edge (+15% Crit Chance, +15% Status Chance). When this was a viral/slash build, I used to run +20% CC. in the new corroblast era, status chance becomes relevant again, and losing a little CC for a large status chance bump becomes worth. We don't want to go full status tho, this is definitely still also a crit weapon.

Fire rate is really, really good; greatly scales the damage output and it's so easy to get back to incarnon there really aren't any ammo concerns. IMO Vile Acceleration is a must-have if you're not getting more fire rate from another source. Note that punchthrough has no effect on the beam weapon (and is actively detrimental to the base weapon), so don't use any [Shred] variant.

Primed Firestorm is also very good. Not only does it increase the base form's AoE, it increases the invisible on-impact AoE of the beam. All targets hit by this AoE take full damage and become eligible to spread the beam to other targets outside the AoE, beginning a chain beam. By increasing the size of this AoE, you not only make it easier to hit enemies in the first place, but you also enable the beam to hit more targets and over a wider area. there are a limited amount of chain beams this weapon can spawn at once, and by increasing the number of enemies inside the initial AoE you effectively let this beam hit more enemies farther out. it is definitely worth the slot. Here, I made a whole video on the topic of how Primed Firestorm affect the Torid incarnon's beam chain mechanics. Alternately, just see for yourself in this interactive simulation I coded.

I have a +CD +Damage -Corpus damage riven, it's not peak but it I swap [Vital Sense] out for it and it performs swimmingly. The stats you want are CD > CC = Multishot = DMG > Fire Rate, basically. Normally multishot would be top dog, but the [Torid] has some weird mechanics going on that results in much of its damage ignoring the stat entirely. Though, when it does work, it works even better than multishot on non-beam weapons, since due to the way they coded beam weapon multishot (it emulates multishot by applying a unique multiplier to damage & status chance), you actually double-dip the stat when it comes to damaging status effects.

As for other approaches, Viral + Slash is still good, and Viral + Heat is newly good too. Viral + Slash used to be way better due to enemies having tons of armor and small HP pools (slash DoTs ignore armor entirely), but the recent update made armor much less impactful and buffed HP to compensate, effectively heavily nerfing Slash. Meanwhile partial strip became much stronger, so Viral + Heat is now competitive as well. But ultimately Corroblast is top dog in my book.

Changelog

9/05/24: Updated a bit to explain multishot a bit, offer alternate build approaches, and plug my youtube video/simulation lol

8/02/24: The latest patch (36.0) completely changed the game, and thus so too has the build. No longer is this a viral + [Hunter Munitions] slash build; both slash and HM received a hefty indirect nerf because slash's perk of dealing true damage that ignored armor was made worse by the update to enemy armor/health values, to heavily nerf their armor and give them HP in exchange. now 'armor bypass' is less valuable, and slash procs effectively deal less damage since enemies have more health now.

The new king on the block is Corroblast; against a max armor enemy (2700, 90% DR) each stack of corrosion outperforms Viral, point for point, and the new blast status greatly amps your damage and gives it quadratic scaling based on enemy density at the same time. pair with a Roar or Nourish buff or +max corrosive stack shards for even more damage.

Viral + slash used to be amazing for chewing through high level armored enemies without needing fullstrip. Pair with roar faction damage and enemies would melt pretty well. Now that they're capped to 2700 armor and their armor formula changed and partial strip is far more effective, Viral + Slash just isn't needed anymore, and by all accounts appears to be outperformed by the new Corroblast meta.

There used to be a whole "flex slot" section of the guide but it's gone now because the build just got a lot more tight. we went from 1 elemental mod to 3, a rough price to pay but ultimately worth it.