To unlock the auto-win button, you have to beat level 3-8 in the Story map of the Adventure mode. You can then use the Auto Win Tickets to obtain Adventure battle rewards without fighting, only on those battles where you've already obtained 3 stars.
Follow the below steps to use Auto Win Tickets:
- Tap on Battle > Adventure
- Select a battle where you've got 3 stars
- Click on Auto Win
- Choose the number of Auto-Win tickets you wish to use along with the Battle Energy
- Click Confirm to get the rewards