Edit: Please note that estimated play time required for a 1% increase will likely be around 60 minutes now. At the time I wrote this solution the game experienced massive framerate drops and crashed frequently, an update has addressed those issues and made this achievement a little less of a grind.
You're going to be in this one for the long haul. After completing the rest of the achievement list, with about 100hrs of total play time, my counter was at 32%. This achievement required about 100-150 additional hours of dedicated play time.
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 levels 60-63, when "Dividing" (enemies have a chance to split into two) is added as the permanent modifier. Get a level with the additional "Gauntlet" modifier (enemies will constantly respawn). For maximum enemy density, make sure you get a level with a goblin pictured on the card, and always choose the cave or underground warehouse levels over the forest one. Cards will reset if you leave the game or complete another Fazeer Dun'djinn level. Afterwards, find a nice route within the level and grind away. I have been averaging a 1% progress increase every 90-120 minutes, which includes load times and searching for a good level, using this method.
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.
Please leave a comment if you have recommendations for builds made for grinding and I will update the guide.