Just so you guys know, the rare boss Anne can drop Wind-up keys. These will turn into potions some time after the event. She's relatively easy to kill, so put your highest event unit up as a lead and skip any boss that's not Anne. The optimal way to farm for rare bosses is to keep your training points near max and take a step every few minutes. When you get fever then use up all your points, and perhaps personal pots if you'd like.
You can also kill the lesser bosses but your fever will run out.
Have gotten at least 2-4 wind-up keys daily without any effort. She can also drop personal pots and mind spanners so you can get the Metal heart rewards. Training stage completion also gives about 5 windup keys in total.