So. The game will let you know when the rites are coming to an end. You'll be told there are two liberation rites left: the one you are about to play, and one more. Prior to the last liberation rite you'll have one last regular encounter. Your rivals won't show up to the contest (happend to me al least while facing the worms teams). So, light up your 12 titans stars and go ahead.
Not sure about this but probably by doing this you can also get de 3/6 titan stars achievement also.
Other than this cheese, if you want to truly beat the achievement I recommend trying against a team towards you feel confident. Use consumables for bonus stats and your fastest team members. I got my 3/6 titan stars achievements having at least Pamitha and Umhilde (flying/jumping abilities with theese two). Also if Pamitha has gone through Sandra´s test you can have her using the feather amulet which allows her to stay in game when scoring while flying thus allowing you to not suffer banishing penalty.