When you are fighting, you will see in the top right corner of the screen a button which will let you turn on the Auto fight in the game. That will automatically use the heroes' attacks and skills when their turn comes, so I suggest that you turn it on in the early stages, since they are pretty easy. As for the later stages, use the skills yourself because you will know when to use them and how to time them right! This is especially good against very powerful opponents! Now next to that Auto battle button, you can also speed up the battle! I found that this is useful, because usually the animations take quite long, and it's easier this way! The battle result will be the same (hopefully you winning), it's just the battle speed that is going to be sped up, which will let you progress faster through the stages.