Hi! This is the best build I've come up with so far for the Endgame [Mesa]. We are using normal [Regulators], because after some tests I've done, it still seems like [Mesa Prime] attacks are bugged and [Reach] the maximum fire rate way slower than normal [Mesa].
The build is made of some core [Regulators] mods and two elemental ones, toxin and electricity, for the corrosive effect.
Why did I choose these mods:

[Galvanized Diffusion]:
this is currently the best pistol mod out there and it has to be put in every [Regulators] build. You need this mod.

[Galvanized Shot]:
again, one of the best out there, it's very powerful considering this build comes with 5 different status effects, and the firerate makes up for not so great status effect. Why is it so good tho? Comparing it with [Hornet Strike]'s 220% damage, this mod gives us, when stacked up, 120% damage for every type of status effect affecting the enemy, which goes up to 120% * 5(number of status effects) = 600% damage. On top of that, it also gives us 80% status chance.
It is completely needed for your build, considering its low price. Don't waste your endo on ranking up [Hornet Strike], save some plat and buy [Galvanized Shot].

[Primed Pistol Gambit], [Primed Target Cracker]:
Two most important mods after the galvanized ones, both greatly boosting [Regulators]' already good base crit stats. If you don't yet have those in primed versions, I would buy the [Primed Target Cracker] first.

[Pistol Pestilence], [Convulsion]:
You need those mod for the Corrosive elemental damage. Corrosive, when fully stacked (10) gives you 80% armor reduction. For Steel Path, where enemies have lots of armor, this is way more important to have than Viral.
Additionally, you should use Nourish on your [Mesa] to make up for lack of damage that Viral would give you.
You could also use one more forma and go for [Pathogen Rounds](90% Toxin) instead of Pisto Pestilence, but I don't think it's worth it since it only gives you 30k burst damage increase (950k) comparing to [Pistol Pestilence] (920k). On top of that you would have lower status chance and your status effects would take more time to fully stack up.

[Primed Heated Charge]:
This is the only Primed elemental mod for pistols. It basically greatly increases damage output, although it's status effect isn't working too good with corrosive, since corrosive usually hits the reduction cap before heat is even applied. Still, the base damage increase with this mod is something you can't replace with any other mod.

Lethal Torrent: (Best in slot - Primed faction mods):
Last but not least, this mod increases both your multishot and fire rate. Considering you use Arcane Velocity for 120% fire rate, your total fire rate bonus is 180%. It means Lethal Torrent only increases your firerate by 27%
(280%/220% = 1,273; 280% = 100%(base) + 180%(additional - arc velocity and [Lethal Torrent]), 220% = 100% base + 120% arcane velocity).
The Multishot increase also isn't all that great, since you already get 230% multishot from Galvanized Diffusion - Lethal Torrent increases it by 18% (390%/330%). It doesn't sound so great anymore, although it still gives you 50% (1,22*1,18 = 1, 442) damage increase for the last mod slot, which is almost as good as Primed faction mods. There isn't any other mod that would boost the damage more.

If you also use [Amalgam Furax Body Count] mod, you get additional 45% fire rate, which further weakends the [Lethal Torrent]. This way, [Lethal Torrent] would only give you 44% damage increase. In that case, I recommend running with [Hornet Strike], since it gives you 55% damage increase when [Galvanized Shot] is fully stacked, and way more damage if [Galvanized Shot] is not stacked up. This mod requires additional forma.

TL;DR:
RUN THIS BUILD ON BASIC MESA. PRIMED VERSION HAVE BUGGED ATTACKSPEED WHEN REACHING MAX FIRERATE ON HER 4

Core mods:

• Galvanized Diffusion,
• Galvanized Shot,
• Primed Pistol Gambit,
• Primed Target Cracker,
• Primed Heated Charge

Corrosive elemental:

• Pistol Pestilence + Convulsion

Best in last slot mod:

• Lethal Torrent
• Hornet Strike if you run Amalgam Furax Body Count

Best to run the build with:

• Arcane Velocity,
• Molt Augmented OR Arcane Avenger,
• Nourish OR Eclipse from Helminth
• [Amalgam Furax Body Count] mod, equipped on [Furax] meele weapon, gives 45% additional fire rate

You should also check out my [Mesa] build