**In order to get Cupido as a summon, you must have chosen the summoner class upon starting your game.** If you are not a summoner, Cupido will not join your party and this achievement will be impossible to get on your current playthrough.
Cupido can be found in the Love Engine Dungeon. At the Love Engine, there will be a chest with a switch handle. With the switch handle, leave and head to the southwestern area of this room. You will come to a switch with no handle. Insert the handle and use the switch to open the gate. You'll come to a room with several switches and cages containing several monsters.
There's a bit of a puzzle here. You must pull the correct switch that corrosponds with the number of water pipes that can be found in the Love Dungeon. There are three pipes, so pull switch #3. Now you can speak to Cupido and get him as a summon.