You gather stamina points either as mission reward or by waiting it out. While you wait on some hero levels or building upgrades to be ended up, I recommend that you take a look at the map and start checking out. Go right ahead and battle it Whenever you see a new monster that you have not eliminated yet! Similar to the monsters that are new, the high level monsters aren't to be disregarded! When checking out the map, they offer great EXP and it's great to take them into factor to consider. If you wish to rapidly charge your stamina, you can also use Diamonds for it, however as I stated, my preferred use of Diamonds is for summoning new heroes, however you can do as you want.