tl;dr Save your gil, farm Roboressurection or Ghost in the Machine until you get item discount buff, then go to Old Lestallum and buy meteor shards.
Fairly straightforward grind, although there are a couple tips that will make this substantially easier.
For the pure grind approach, there are two missions that are quick and easy to complete: Roboressurection (lvl 25) and Ghost in the Machine (lvl 39). Each requires just taking down one robot boss, which can be done in just a minute or two with a decent character, and awards ~6000kW per run. I'd recommend using your strongest spear with the Aerial Ace sigil to get easy breaks.
Tip two: save all your gil and wait until you get a Cindy item discount buff post-mission. Once you have that, travel to Old Lestallum, run down the stairs opposite where you spawn in, and head to the shoreline. The fisherman there sells meteor shards that convert to kW. It's roughly 225k gil for 500k kW (half what you need), but if you have the discount buff, it drops to about 180k instead. Combine with some light grinding and you'll unlock this in no time!
One last note - I've heard you may need to hop in to the power map to actually get the achievement unlock, although I personally didn't.