First you need to unlock the box, 75 is enough. I have been practicing until 100. It is yellow. You'd better practice higher. Practice at the farm next to Hubin Town. The key is to steal that lazy thing from him. Stop fighting, it's stealing! ! ! !
If you make a phone call, you may hang up. Although he is green, it hurts ~! ~~
After stealing, you will have the key. The key is the task item, which has no value. Unlocking skills are the key. Just go up and open it.