Mimics are common in many games as a monster that disguises itself as a treasure chest. Most of the time in RPG games, they are easy to dispose of but not in the Alternate World. These little buggers are very rare to find in the game but if you ever find one, it is considered a challenge to veterans and an absolute disaster to beginner players.
They don't possess high health but they possess high defense values and have a very huge resistance to both physical and magical skills which means they are not weak to anything. Since they have around 900k for its HP, it will take a good amount of time to kill it. Of course, it can be cheesed with destroying the chest itself by bombarding it with explosive skills that can either pierce defense or other skills that ignore defenses and break it immediately, if you don't need the treasure inside the Mimic itself.
