Guide needs to be updated, but this build is still what I will use for the foreseeable future.

[Mesa]'s main damage source comes from her 4th ability, Peacemaker, so this build focuses on making that ability as efficient as possible to use. What do I mean by efficient? I don't mean Ability Efficiency, I mean KPS efficiency. Regulators can output some of the highest non-AoE DPS in the entire game, so the main way to increase [Mesa]'s KPS does not come from adding more damage, but from how fast you can acquire targets. With that in mind, let's go over the mods.

Holster Speed:
Aside from the ridiculous energy drain, Peacemaker's main problem is that there's a delay between pressing the ability button and when you can start shooting. Without any mods, you'll notice if you press 4 and then LMB immediately after, [Regulators] won't shoot. This is because of the ability's casting animation, which is a holstering animation. While [Natural Talent] can help with this, Holster Speed is much better. The 220% faster swap speed when Speed Holster and Vigorous Swap are combined is much faster.

Mods:
-Speed Holster: If it wasn't obvious already, Holster Speed is one of the best things to use on Mesa.
-Augur Secrets: This mod will allow you to reach the 95% DR cap on Shatter Shield.
-Streamline & Fleeting Expertise: This will max out our Ability Efficiency stat making Peacemaker as cheap to cast as possible.
-[Primed Continuity] & [Augur Message]: These mods nullify the negative Ability Duration from Fleeting Expertise and increase the uptime on Shatter Shield and Shooting Gallery.
-Primed Flow: A larger energy pool will allow for more Peacemaker uptime.
-[Rolling Guard]: When your shield gate breaks, this will give you 3 seconds of invincibility allowing you to gain back shields using the Augur set bonus and reposition.
-[Vigorous Swap]: More Holster Speed to shorten Peacemaker's casting animation. The +165% damage is also a somewhat useful damage boost for Regulators.

Exilus:
There are 3 options:
-[Preparation]: [Preparation] is for shorter missions where you won't be able to find much extra energy. In a mission type like Survival, the amount of enemies being killed should give enough energy orbs to upkeep Peacemaker with a little bit of help (Energize, Zenurik etc.....). Important note about Preparation: Never level this mod to max rank. Rank 8 will still give you max or near max energy.
-[Primed Sure Footed]: While you are immune to knockdowns while using [Regulators], you are not when you are moving, so this mod removes unnecessary annoyance, increases KPS and gives you better survivability by keeping you off your ass for 99.99% of the time.
-Bullet jump mod: This allows you to cover more area and get higher in the air so Peacemaker can cover a larger area and kill more people with one cast.

Arcanes:
-Arcane Velocity: This is a "must-have" on mesa. It is a massive DPS increase.
-Arcane Energize: I find this is helpful with keeping your energy at full, but it can be replaced with most anything else. Especially is you subsume Dispensary.

Helminth Options:
This build works without any Helminth abilities, but I could highly recommend using Dispensary with [Equilibrium] and [Synth Fiber]. With [Equilibrium], you will get at least 135 energy back per cast, and that's only if you only stay around for the first rounds of drops. If you do subsume Dispensary, you will need to add another Naramon polarity and replace [Augur Message]. I would also drop Arcane Energize for something else.
If you don't have [Synth Fiber] or you don't want to use it for whatever reason, you can replace the Aura with [Combat Discipline]. While this does make casting Peacemaker significantly less smooth, this will allow you to consistently proc Arcane Avenger, which can be a pretty nice DPS increase.
In terms of other options, Infested Mobility is nice for the extra Parkour Velocity. This will allow you to drop the bullet jump mod if you used it.

Regulators Prime Build