About the Unite cup event I suppose that the opponent level depend on the medium level of the association teams.

About winning with a +10% stronger team you should start thinking as Nordeus. Forget Topeleven as a real footbal manager game.
If you wanna win you have to pay or exploit the known tricks.

In my current friendly championship there is a 98% team on top and there are 4 120/125% teams in the championship.
The trick? It's simple. It has 3 monster players (over 160%), 5 medium players in the range (100 / 120%) and all the others players 1%.
With this team he won every match against 120/125% teams.
According to Nordeus, if you have Ronaldo and Messi in your team you can play and win only with 8 players.
Thanks Nordeus