You're going to be in this one for the long haul. After completing the rest of the achievement list, with about 100-150hrs of total play time, my counter was at 32%.
Difficulty does not affect this achievement, so play on practice to blast through enemies. The best place to farm enemies will be at the Fazeer's Dun'djinn dungeons that have the "Gauntlet" modifier. Enemies will respawn constantly and you won't have to deal with load screens, making the grind time a little less. Every class has good builds for farming. I prefer the Sharpshooter with a build focused around the skills "Loyal Shasta", "Targeted Strikes", and "Scattershot" using the weapon Wingspan. Pick a build that works for you and grind away.
Please leave a comment if you have recommendations for builds made for grinding and I will update the guide.