A Mimic is a monster that looks like a treasure chest. You cant tell the difference without opening them and causing them to attack. There is a way to beat them without awakening them. When you come across a large number of treasure chests in a dungeon, they are most likely mimics. To defeat them simply perform windmill (or another AOE skill like a.. Shockwave, Bullet Storm, Shooting Rush) in front of them as it is the only way to hit them without opening them.
There is another way to beat the mimics without having to fight them. If you come across a large group of them and have need of finding a key use a mountable pet.Simply click on the chests until you find the one with the key.But do NOT dismount because the chests you clicked on that are mimics will awaken and come after you as a group.
And you can use "Lullaby" to make it fall sleep which will show the chest with a key.