I finally made it to the end of Angry Birds Epic. The last few sections were extremely difficult, and I had to do a lot of grinding. Fortunately, the game has an autoplay feature. I just set my party to fight the battle with the most enemies over and over again for several days until they leveled up enough.
The second-to-last fight was pretty unfair, in that you had to fight three bosses at once. In addition to this, three healing bombs show up every third turn. If you don't kill the healing bombs right away, they explode and bring the bosses back to full health. In other words, every third turn of the battle is kind of wasted. You can't use it to attack the bosses; you have to use it to stop them from healing.
The last fight steps up the unfairness. The boss has a move that allows it to heal all of its health. And that's just phase one of the battle. Phase two is the one where the boss has a move which does 550 damage to all enemies. That narrowed down my options for the battle, since not all of my characters had more than 550 HP. The boss' healing system in the second phase was pretty unfair--at minimum, it healed a fifth of its health each time--but it was easier to circumvent than in the first phase. I still think I got lucky, but I'm not complaining! I managed to beat it!