After finding challenges and PvP very easy for a couple days, I have discovered the app has become completely unplayable as of 12:00 am on January 9. Since then I have been facing nearly impossible armies in the Challenges. The challenge rating of my foes is so high I’ve even had to surrender in the 1st room. There is no longer any point in trying to repeat these rooms.
This is insanity, room after room of too powerful enemies. Hopefully this is a short term glitch or something they are working to resolve.