Complete the following tasks to get the monster... (You get the monsters from the helpers in Dr. Snapps office at the very end)

Empyrea: Get all monsters.
Grandpa Slime: Get 200 monsters.
Leopold: Get all skills.
Liquid Metal Slime: Get 100 monsters.
Metal Kaiser Slime: Get 150 skills.
Metal King Slime: Get 100 skills.
Robbin' Hood: Successfully complete the monster and skill libraries.