There are 5 difficulties and you need to complete 3 of them each.
Easy, Middle, Hard, Expert, Master, Grandmaster.
You CAN do failures, just enter any number until the correct one (it will turn blue).
You CAN activate all helping tools to make it quicker.
Should take you about 30 minutes - or even quicker, if you just put random numbers everytime.
You can check your progression on the achievement list.
Remember if you leave the game, you have to start over. So do this after you completed a puzzle