I messed up several times in the beginning with hiring 2 heroes at once. Then I tried a new method: I hire only one hero, buy Lvl1 for Health, Skills, Wisdom and Lvl2 for Gold. This way the hero earns enough gold to buy HP potions permanently and even save up some money. Now I can slowly level up all the other skills to Lvl2 at least - if possilbe Lvl 3 - and hire further heroes.