Personally, I find the difficulty to be in a good sweet spot that strikes a great balance between newer players and seasoned vets. Of course it's going to be easier if you use a hero almost designed to fight a specific enemy, and play by yourself.
Try solo'ing Ultron with Spider-man. Or take Iron Man against all 3 villains in a row on expert and record so we can see the master in action. It's entirely possible that you are just really good at the game and are ahead of the average skill curve.