In this video I show how I got the mimic to spawn. Pretty much all you have to do is go into the main event and start playing 15+ zone missions in Save the World, and during those missions open up chests until you can get the mimic to spawn. I'm not 100% positive, but I'm pretty sure the tier 3 chests (which spawn the mimics) are most often found in Canny Valley. You know you've found a mimic chest when it is shaking.
