Madrid toughest opponent is Barcelona who have already won the trophy 14 times and have many star players. Their win against Mallorca has given them the mentality to win the final. If they win the final they will be level with Barcelona trophy won. However I will put both teams ahead of them because their strength and ability are almost the same which is a huge competition on the field.
Yes we can say that Real Madrid's toughest opponent is Barcelona, but currently these two teams cannot be called tough opponents. The reason is that Real Madrid is currently stronger than Barcelona, Barcelona football team was much stronger in the past then they fought equally against Real Madrid. There has been a lot of difference between the recent performances of these two teams, the way Real Madrid players have performed in the last few matches, maybe Barcelona players have not been able to perform in the same way.
I somewhat agree with you that Real Madrid has been much better than Barca in terms of performance in recent times to win the trophy.
Actually, it can be very difficult for us to predict which team will win the trophy in this match, because both these teams have some players who can suddenly give their team victory by performing well. You may have noticed earlier that Barcelona beat Real Madrid, even Real Madrid didn't score any goals in that match.