With the release of 7.0 Legacy of the Sith, Bioware has added 2 sneaky little achievements for C2-N2 and 2V-R8, each achievement grants you a FREE companion customisation for the droid companion.

To earn these skins, you need to kill 500 Black Sun, Exchange or Hutt Cartel NPCs with both C2 and 2V. I suggest using a Rampage Achievement Boost that you can gain from your Login Calendar Rewards as it doubles the count of all kills made.

The best place to farm these kills is most likely on Tatooine outside the Imperial Heroic 2+: War On Spice.
This area is covered in low-level mobs in groups of 3-4 that respawn very quickly, as it is considered a quest area.
Farming in this area with both companions and a boost, I was able to complete both achievements in under 30 minutes using a ranged DPS class and 2V/C2 set to DPS.