This encounter requires you to have the Eternal Hope blessing, which you initially construct in the Devil challenge, but can optionally be assembled anywhere after getting the gold for Devil. I recommend trying for this achievement in Devil anyways, since it is otherwise hard to get the 3 blessings, 2 required encounters and the ring to summon Theo all at the same time, and the game more or less sets it up for you.
Load up The Devil with Dealing with Thieves, and Forgotten Dreams in case you run into Dealing with Thieves on the first floor. Forgotten Dreams seems to be seeded for me to always appear on the last level of the Devil, on the very bottom square of the path you can walk on. Make sure you have Eternal Hope before you trigger Forgotten Dreams: Once Forgotten Dreams finishes, you will be transported to a still-active encounter, giving priority to unexplored ones. This can mean you shut yourself out of getting the achievement if you didn't get Eternal Hope yet.