I would say that the leveling system of this game is so easy, from hiring workers, planting harvesting crops, putting up building and other infrastructure of the game lets you gain more EXP that will surely put you to higher. But before that fill out the amount of EXP level you need to level up. So the more you harvest plant and do other faming stuff the more you gain EXP, gold, hearts and pearl which you will use along the way as you keep playing on this game.
Aside from this, going in to the battle will gain your hero more HP, Attack and defense on winning on the battle.