Hello,

The faction and elemental mod choices are specific to The Index, but they can be easily swapped for other applications. Index enemies have alloy armor, which has a -75% effectiveness against radiation.
Due to the way damage is actually calculated, the result is far more than just 75% extra damage. For an enemy with 600 armor, radiation will do 3.5x damage compared to a neutral damage type (like blast) and 9.3x the damage of a weak damage type (like electricity).

Why not use Hornet Strike or Galvanized Shot?
Mesa's exalted ability renders them sub-optimal. The specifics are incredibly wordy, but the gist is that they get a buff upon being drawn and when fired, which is additive to damage mods such as [Hornet Strike] and [Galvanized Shot]. The buff is affected by ability strength and can [Reach] absolutely insane numbers. With a build like this, giving 238% ability strength, the [Regulators] get a 714% damage buff before even firing the first shot, with the 20th shot receiving a 1785% buff.

Adding 220% from [Hornet Strike] or even a whopping 480% from [Galvanized Shot] on a target with 4 unique status procs will only increase the relative damage output by 12% and 27% respectively.
You will see more damage, much more reliably by using faction mods, even un-primed. They are multiplicative, so even a basic "[Expel Corpus]" mod will increase damage by 30% and it will affect every single shot.
A primed mod can be added with the use of another Forma, which I strongly suggest doing when you can. The extra damage is utterly ludicrous.

Why use Anemic Agility instead of Gunslinger?
Same reason as above. the -15% is additive (subtractive in this case) to the damage buff mentioned above, and it's utterly irrelevant compared to all that extra damage. The extra 18% fire rate however, is not. The damage scales with every shot so more shots per second = more DPS.

Go forth and aimbot!