If you are stuck on campaign level, try different troop setups and upgrade your hero level, grade and runes. Remember, that you can "try again" stage for free and you dont lose your energy. This is great as you can try different tactics. I would also suggest to use Saoirse in campaigns because of her AOE heal.
So far I have done 7 chapters on normal and elite and I have used only sunsinger soldiers as my backup. You can try to build different clan buildings aswell to try which one works the best. Clan buildings boosts your backup soldier stats (atk, hp, def and the amount of backup soldiers), so remember to upgrade those. Personally I have used Solar tower, Steam engine and The guardian.
I remember, that chapter 7 elite levels were quite tricky. There was few battles where you really had to hone your strategy to get 3 stars.