Tasks are the easiest and fastest way to level up. They are also a great source for mojo, gold, and crystal since each task gives you 1mojo and steadily increasing amounts of gold, crystal, and experience.
Succubus- For the succubus I have found that once you have access to the wolf rider, the most effective formation is a row of 6you knights in the front, a row of 6 mages behind them, a row of 66 wolf riders behind them, and finally a row of 46 wolf riders and your hero(with your hero in the middle of the row). I have used this formation for every task from "A huge wave of enemies" to the last task I completed which I believe was called "Showdown with the jack brothers" and since it shows no signs of loosing its effectiveness i plan to keep on using it.
Other heroes- sorry i cant be of much help since i dont have them :/