Make sure you have The City DLC purchased, or that it's on rotation for free trial, and activated for the game. Choose the Prophetess as your character. Add a local player that you can control, and set them also as the Ninja character.
With the Prophetess, enter the City and focus on acquiring Gold. She can also use her ability to replace the immediate card you draw, giving you an advantage.
With the Ninja, make sure you lose at least one life, but try not to go below 2-3 in case you accidentally draw the Pestilence card, or similar. Preferably keep your life at 3. Then just move about, Evading any encounters with enemies, and otherwise not picking up anything. Just do as little as possible with the Ninja. (The Ninja is good for being able to evade all enemies, and not being able to pick up Followers.)
As the Prophetess, keep running around in the City area, until you eventually draw the Assassin card. Spend the maximum of 3 gold and target the Ninja. The Ninja will lose all their remaining 3(or fewer) lives, and be killed. This will unlock the achievement.
The time it takes to do this will vary greatly depending on the luck of the draw. It took me about 15-20 minutes.