The only reason I use an Aura and Umbral Forma in this build is because they are required for my alternate [Mesa Prime] Gloom Shooter Build. If you do not wish to spend your precious Umbral Forma or an expensive Aura Forma to do my Alternate build, instead do an additional Vazarin - D and Naramon - Dash/Bar Polarity instead for the same 3 Forma cost.

Aura is of course personal choice. I like [Pistol Amp], but you can also run [Growing Power] if you'd prefer the additional Ability Strength (Not like [Mesa] needs it to be quite honest).

FAQ
Why Mesa's Waltz in the Exilus slot?
Mesa's Waltz does a lot more than just letting you move/roll around while using her [Regulators]. It also ignores the wind up animation of her [Regulators] letting you begin turning the map into an Old West Tavern faster, while also allowing your focus ring reticle will grow back gradually when not firing.

What about her pew pews?
The [Regulators] build I use for [Mesa] is a standard Viral/Slash/Heat build.