I dont know if this is a bug or intentional. I think it starts at stage sixteen. You just have to defeat the waves until lets say wave 8 and let your gems be stolen. You kill about 90% of all enemy. Plus 25% exp bonus, thats more than 60k exp. So you dont even get close to win that map but can level up twice...
 "Cursed Treasure 2"){kind=icon}
 "Battle without End"){kind=icon}