Things You'll Need
Instructions
Walk around the map and battle the random monsters and trainers that appear. This will increase the class of the monster you battle with, which should be the hopper monster. Train the hopper monster up to B class or higher.
Allow the hopper to dig up items during December, January and February. The hopper will eventually unearth a hot spring and an undine "slate" item. The game will notify you that you have received it.
Travel to the combination temple and speak with Dadge. Select any two monsters you are willing to give up and select the undine slate. The monsters will be combined into one undine.