Challenge matches are broken
While challenge matches are a fine idea to protect players who are not very good, it ruins the game for moderate to good players, here is the reason why:
As you fight tour way Up through the ranks, you end up only playing challenge matches, thus never loosing any medals, which result in your rank/league not being regulated down when you loose battles.
In the end you will loose more and more battles but still rise in rank/league until you only play against players who have units that are 3-5 level higher and are 20-30 player levels higher.
There are no way to lower your rank/league, even though you loose 99% of battles, because you only play challenge matches.
Solution:
Make it possible to disable the challenge match feature, so even though you battle a high level player your rank/league will be regulated to match your level of play and unit level.
Hopefull regards from a player who just entered master league at level 30 with lvl 8-9 units (unit Max level are 13 at master league).