That's not entire true. I encounters this issue and then I fixed it. U need to rank up. Until u unlock at least 4 heroes. The healer and dwarf has 10 stamina. You can group them together. And the other initial 2 have 5 another team. If you been hitting the enemies using either one of the team u will notice u can control when they level up etc and thus refill your stamina. Almost never running out of stamina.