Make sure you have The City DLC purchased, or that it's on free trial rotation, and activated for your game. Choose the Bounty Hunter as your character. Choose the second player as a local player you control, and make them the Druid.
Get a character bounty card from the City(or by getting it via an adventure card). The character bounty cards specify that you need to take one life from another player character of either Good, Neutral, or Evil alignment.
Use the Druid's special ability to change into the alignment necessary for the bounty, then encounter each other(doesn't matter who attacks). Win with the Bounty Hunter and take the life of the Druid. This will activate the bounty card, and you now have to make a roll.
If you roll a 6, you gain that many gold from the bounty, and the achievement unlocks. If you don't roll a 6, use a Fate point to reroll. Once you get a 6 on the die and click to continue, you will unlock the achievement.
However, if you still don't get a 6, immediately exit the game, and then resume it from the menu. You should be back in the game before the Bounty Hunter and the Druid encountered each other, so you can try again. (Keep in mind that you'll likely roll differently for movement, and it will probably take some turns before you encounter each other again.)
Should be doable within 10-20 minutes, but there's a lot of luck involved.