The most important thing that I missed as a newbie was the ability to run away in the middle of a fight. That should go near the top of any beginner's guide:

"If you see that you're going to lose a fight, tap the red X in the top corner of the screen immediately! This will save your remaining hit points, and your progress up to that stage. You can wait for your hit points to fill up and return to the same fight. (The monsters will heal, too, but you won't have to re-do the earlier stages.) If you had friends with you, they won't heal, but you won't need to re-hire them. The special attack bar will retain its fullness, so you can also use this to make sure you get the drops from a boss: if you see that your bar won't be full when you reach the boss, kill minions, run away, then come back and kill the minions again to fill your bar.

Important note: if one or more of your knights are dead, they will not revive for that fight until your other knights in the fight reach full health."