It's been stated by saltypatra that the game needs to be fair but challenging but the problem with that is that this game is a casual grinding game and not a competitive one.
Ranked PVP is a grind not a competitive mode even though it has a leaderboard as last weeks winner completed nearly 3000 battles so at 2 minutes per battle average that is around 14 hours per day of playing PVP every day of the week so how does one compete with that.
GW goes a bit towards being competitive but it has divided the community a little bit so I'll leave that one alone but I think it shows that the game is more a casual one rather than a competitive one.
The parts of the game I love the most are getting those gem cascades in my favour and winning battles because it is a grind, it creates a nice feeling which is why I keep playing plus I love the social nature of the game with the guild system. If I keep losing battles to bad luck then the pyschological impact is not good when I am spending a lot of my time grinding but not actually achieving anything.
If you want to create a challenge, then a survival mode where highest amount of enemies killed wins each week or a weekly leaderboard for treasure hunt, not base it on how many hours you can put into the game each week. You only need fairness to both AI and player if it isn't a grinding game but this game is a grinding game so you need to weight it heavily in the players favour or have a completely separate competitive mode that is optional for those who want the challenge.
To me that is an easy way of keeping both sets of players happy (casual and competitive) as at the moment I have a 1000+ pc player in my guild ready to quit due to the game not being fun any more purely becuase his losses have sky rocketed and that doesn't create a nice feeling for him. Fortunately I play on mobile so not a real issue for me at the moment but we will see what happens down the road