No doubt, it is to kill more enemy Heroes in a stage, while purposely losing it through time count down. After raising your Leader Hero to such an extent that the Hero is at least having 5 levels higher than the enemy Hero's, now focus to kill the Heroes in the stage. Remember, do not win the stage as the enemies (both army and Hero) will grow stronger. Patiently wait them to get spawned and kill them with the tactics above. Killing one Hero may reward you around 200-500 experience, with around 1000-2500 Golds, plus 1 possible treasure that may contain up to 2500 Golds. Once in a blue moon, the treasure may come with 1 blue Gem too (as in the picture, the crate is Gold in color)! Until now, I have broken my previous records and now obtain more than 30,000 Golds in a single stage, though 'losing' in the end. Repeat the process infinitely, you can even train more Heroes to achieve the maximum level of 45.