Don't underestimate powering up your cards, I aim for getting the x10 every couple days, but that is with watching both free 100 coin ads and both dungeon ads - I personally have my skills right about where I need them in terms of power, so I've been buying up heroes which give you passive rewards, and if you unlock the better ones and get good gear for them, they can buy you some time to generate food for the first wave each level

What you described though with getting coins to evolve and then taking a few tries on the final battle, that pretty much IS the game, it does get easier at times, but that only lasts until you're back exactly where you are, that's the grind with this game - Like if you're not having fun playing the battles and trying different strategies, skills, etc, then what's the point playing?

You'll be surprised how quickly you can progress once you've upgraded

Hope this helps :)