There are those sort of "benchmark" levels that all players get stuck on, and they force you to farm and build up your creatures to be able to move past them. Just give it some time and work on getting your team stronger, and you will get there in time.
I wouldn't focus so much on going through the levels. Enjoy the other game modes and build up your team. The levels are just one game mode among many. There is no special price to win when finishing all levels. I think most people think they "win" the game when they finish all levels and are disappointed when nothing happens. Go through the treasure caves and replay old levels to get gems. Every once in a while try the tricky level again when your creatures are stronger. At some point you will be able to defeat the boss.
I don't think it is you that needs rank 4. It is the creature. So get gems for the creature until you can level him up to rank 4. After that you can level up abilities again.
Comments
Following! I'm stuck too. Got boss down to 50 and WHAM. Dead.
Who are you using?
Compare to all the The boss, these boss life is much more then the player life and i have been switching all creature and still can't be defeated.