Some bosses will seem impossible to beat sometimes (and they are). That's when you hit a wall and the only option you have is to go to the previous stage and grind a lot until you can upgrade your Heroes and Magic Damage more and take out the boss. That's the only way to get over a wall and I recommend simply leaving the game for a couple of hours when you do so in order to gather a large amount of money for upgrades.

If the battle is tight, you might still have a chance to defeat that particular boss. In order to increase your chances, always go to battle with all your skills ready to use and ideally with your Heroes' health at maximum in order to avoid losing them during the boss battle. Use your special skills and tap like crazy: this will help you easily defeat bosses!