This can be easily done with the scripted introduction to Treasure Demons during the 2nd Palace (Museum). This first encounter is with an enemy that is weak to Nuke & Psy.
It's likely that none of your Personas have these elemental attacks at this point. Before going up the stairs to the second floor, toward the end of the first floor, right after the first Safe Room is an exhibition hall. There is a door adjacent to the Safe Room, near the entrance to this exhibition hall, which leads to a long hallway with an enemy and a chest at the end. This chest contains an Atom Ring, which will give Joker a Nuke move when equipped.
After obtaining & equipping this ring, proceed to the next floor where you go through the scripted sequence introducing you to Treasure Demons. Once you start the fight, use the Nuke move "Frei" to immediately knock down & recruit the Treasure Demon.