Torchlight III

Choose the Railmaster, put 10 points on Build Train and Shocking Round; put 6 points on Mortar Car (rank 2 reduces ShokingRound cooldown), put 6 points on Ghost Train (rank 2 increases train attack speed); select only Build Train and Shocking Round as active skills. I have not used any passive ability of the relic as a precaution I do not know it will help or not to kill faster.
If you've unlocked Arc-Powered Pants (+ 25% bolt damage), Arc-Powered Shoulders (the active skill - Charge Up, helps you grind kills faster) and any other Arc-Power armor piece, equip it and add Charge Up as an active skill. in case you could choose bonuses in the legendarium like: Arc-Powered Pants (+ 25% bolt damage) and Arc-Powered Shoulders (active skill - Charge Up), Woodsbeast Sandals (increse critical chance and attack speed by 25%).
Use a weapon with electrical damage and Shock probability. start running next to the Voltura let the train do all the dirty work, use the 2 active skills as you like.
if you are over level 60 you could farm starting levels in Fazzer with Voltura as enemies (I have level 6 Fazeer with Voltura in "The Switchback Witout Bounds" level 51 enemies, my weapon has + 69% stats and running all over the map I reach about 5% kills required per run in about 10 minutes) or you could opt to run in the Grand Hall or Scavanger's Trail.
I hope my information can help you... Good game and good farming...

This achievement is a bit confusing, but hopefully this helps clear things up.

While you can certainly farm kills with Shocking Rounds using the default attack, it's very slow and does very little damage. Despite what the earlier guide here says, you CAN use the tier 3 attack that pairs the Shocking Rounds attacks with the bonus lightning bolts. This makes the grind way, way faster.

Note, when you activate Shocking Rounds you get about 8 seconds of time for when the skill is actually active. You can tell it's active as the little lantern on your car will change from yellow to blue. Once the skill deactivates the lantern will go back to being yellow and you'll need to wait about 12 seconds until you can activate the skill again.

I've been farming this in Act 3, Grand Hall. There are a ton of Voltura. Just park your car, run around and aggro nearby Voltura, get them in range of your car and activate Shocking Rounds. With the tier 3 attack, you'll make short work of them. Rinse and repeat.

Also bugged? My progress stuck on 1% !?!

Edit: Got it running. You must not activate the 3 bonus in the Shocking Rounds skill, then it counts the kills.