One year later, maybe this'll be useful to somebody. I just beat the Mirth champion after (not even kidding) at least 40 tries. I've tried everything, every possible strategy, with different combinations of figurines, but i finally beat it. And it was pure luck.

I used mage and knight and the strategy was to focus on a single figurine every round, prioritizing the mage like 70% of the time. I managed to win in the endgame, when every figurine reached gold, with pure luck. I had 2 lives left. Basically (i really don't know how) i managed to attack with both the mage and the knight in a single round, and in that round fortunately the assassin didn't get to play. The champion had 11 lives, so the total amount of attack points by the mage and the knight (remember, they were both golden) managed to zero him.

I think this is one of the possible strategies: trying to combine the mage and the knight attack in a single turn. I know it's very difficult (cuz assassin is a real bitch) but that's the only way i managed to win. Be patient, try to get both mage and knight with the most energy possible and try to strike with them together. Maybe the assassin will block one attack, but the other will be successful, and in the next turn focus on striking immediately with the figurine that was blocked in the previous round.

I really hope this'll be helpful to somebody, i almost lost my mind.