About Keys and how to select the stages:
1. First click on the marked icon:
2. On the second screen you can select the Manyu Type you want.
3. On the third screen you can select how many keys you want to spend. The more keys you spend (max is 10) the more Manyus you will get.
If you are lucky you will get 20 Lvl.100 Manyus and 30 Lvl.20 Manyus. In my case right here i have only got 10 Lvl. 100 Manyus.