Hello everybody, I'm IbnBattuta and this is my first build on this site, so I hope it helps you!

After testing a lot of elemental bonuses, I found out that each Index type of enemies is valuable to some element while resisting the other, and I'm gonna explain what I mean.

Humans: (Large Shield + Low HP)
Magnetic and Toxic (Gas) are the elements that they're so valurnable to because Magnetic reduces their shield by 75% when it procs which allows you to expose their HP faster while Toxic does direct damage to their low HP, which kills them without having to deal with their shield.

Robotics: (Low Shield + High HP)
Viral and Corrosive (Heat) are the elements they are valurnable to, since they've low shield, you've to deal with their large HP, and that's when Viral comes, which reduces their HP by 50% which means that they gonna die faster, and since they tend to have high armor as well, Corrosive or Heat will make sure to reduce their armor, and allow you to deal more true damage to their HP directly.

Now here comes the problem...
You can't have as many element bonus as we wants, which limits our option to Viral+(Heat/Radiation) or Gas+(Magnetic/Electricity) in which case, we will be able to deal with one type perfectly while having bad time vs. the other one.

Viral/Heat combined is the middle solution for both problems, since viral is a must vs. Machines while Heat will help reducing Humans' shield faster (since it can't directly deal with HP such as in Toxic) and this is the setup that I found that can deal with the 2 types (and there is a 3rd one) in the shortest time possible.

As for why I chose these mods...

Vicious Sprea + Heal Chamber = 3 separate waves from [Arca Plasmor] instead of one.
I don't know why this happens, but it seems that spread when it reduces the weapon's accuracy, you're able to see those attacks separate which you can see the effect of [Hell's Chamber]'s multishot which release 3 waves in all times as long you've [Vicious Spread] equipped.
More waves, more status, more damage, more "spread" so no need to calculate the outcome, it's big.
(Also let's not forget Vigilat Araments)

[Incendiary Coat] is extra to boost the damage for Heat which boosts the damage of Radiation in the same time, while [Blaze] increasing this damage more for both as well.

I didn't go for 99% status, or 93%, and kept it at 81% because [Arca Plasmor] isn't a regular shotgun, so it's not like we need that 100 % before multishot to have 100% to apply 1 status per bullet, what I mean is, your shot has 81% to apply one status, which happens most of the time, so no need for more, damage is better at that stage.

It takes 2secs for enemies in Index to regain their shield back, and that's why Chilling Reload is a must to not waste our damage/magazine.

Last thing, [Fatal Acceleration] is so amazing, since it extends the range of your [Arca Plasmor] bullet waves, and make sure to not have reduced damage from previous falloff range, so it's useful in different cases, and having [Carrier Prime] with you, will save you from having you out of bullets, if you are the guy that keep firing shotguns like machine guns regardless if the bullets hit or not.