You don’t need to buy any levels. You can get all free rewards without spending any gems.
I think you need to average 7 per day to collect all rewards, so you can still miss a few and by the end of the event, be able to collect all free rewards.
You get 10 Battlecrashers every day. Each Battlecrasher gives 2 Crowns.
There’s a counter in the Kingdom Pass that shows how many Battlecrashers you have collected.
If you lose a battle, the loss won’t count against your daily total and another one will show up.