Viral damage and Heat damage have +75% and +25% bonuses respectively against cloned flesh.
This is the best combination for use against armor stripped enemies, but is still good against the widest variety of enemies even without armor strip. This is because Viral status increases damage dealt to all health by 2x after just one stack and applies to all types of armored health including ferrite and alloy grineer/corrupted or unarmored enemies such as corpus, infested, murmur etc.

MOD BREAKDOWN

• Galvanized Diffusion - mandatory multishot multiplies damage by up to 3.3x
• Galvanized Shot - this gives +120% damage per status which is additive to the damage bonus provided by ability strength. Having a maximum of 5 statuses (IPS + heat + viral) after a few seconds of shooting gives +600% dmg, which is approximately a 69% DPS boost. Having just 3 status will allow Galv Shot to be better than Anemic Agility or another element mod.
• Primed Pistol Gambit - mandatory CC
• Primed Target Cracker - Mandatory CD
• Lethal Torrent - gives fire rate and multishot.
• Deep Freeze + Pathogen Rounds - gives raw Viral Damage which has +75% bonus against cloned flesh (regardless of armor status), and +50% bonus and corpus flesh, as well as possibility to give viral status which multiplies all damage done to health, by x2 at 1 status and 4.25x at 10 status.
• Primed Heated Charge - gives raw Heat damage which has a +25% bonus against cloned flesh and also can create heat status effects which deal damage over time (only contributes about 6% of total DPS to regulators due to the insanely low status chance).

Flexible Options:

• Lethal Torrent vs Primed Expel :
  Lethal Torrent is 21% better than Primed Expel if you DON'T OWN ARCANE VELOCITY but only 3-5% worse about than Primed Expel WITH Arcane Velocity. Note Lethal Torrent is effective against acolytes and thrax while Expel mods do not affect them. If you want to min-max Regulator damage against a specific faction eg in disruption, then you can use Primed Expel over Lethal Torrent.
  Lethal Torrent is always better than NON-PRIMED EXPEL.
  Lethal Torrent is better than Galvanized Shot above 335% ability strength when accounting for 5 status effects.

• [Anemic Agility] if you don't own [Lethal Torrent] or Galv Shot.
  Galv Shot is 21% better than [Anemic Agility]
  Lethal Torrent is 7% better than Anemic Agility WITH Velocity but only <1% worse than Anemic Agility WITHOUT velocity.

• [Creeping Bullseye] if you don't own Primed Pistol Gambit.
  Primed Pistol Gambit is 4% better than Creeping Bullseye (with velocity) and 10% better without velocity.

• [Hornet Strike] is a last resort if you don't own [Anemic Agility] or [Galvanized Shot].
  Hornet strike is the worst option, being 8% worse than Anemic Agility, 16% worse than lethal torrent.

Variants:

For Acolytes/Thrax/Deimos Infested or Nourish Setup: Swap [Deep Freeze] for [Primed Convulsion] to make Corrosive Heat
For Corrupted: Unchanged
For Corpus : Switch [Deep Freeze] for [Pistol Pestilence] and [Primed Heated Charge] to [Anemic Agility] for pure Toxin (Bypasses shields)
For Murmur: Swap [Pathogen Rounds] and [Deep Freeze] for [Anemic Agility] + [Primed Convulsion] to make pure Radiation