You need to be at least level 20 and have a blacksmith as your companion. To find the mimic, you need to kill a boss in any of the five areas that has 2+ bosses (you dont have to fight them both). After defeating it, talk to one of its heads and let it decide which way the battle should go. If you win, she will say Its alright, after which point she will turn into an egg with eyes on top and crawl off-screen.