What is the source of the Mages’ power?

"In 4 stacks": this means you divide them into 4 parts. Regarding upgrades: battle mages shoot and attack with their fists, archmages shoot and attack with fireballs. You can shoot first, then attack with spells, or vice versa – it all depends on which neutral creatures you are fighting at the moment. Regarding gremlins: their strength lies in their numbers and their ability. Djinn are usually only needed in the final stages: to cast dark spells (against enemies) or light spells (on allies), to reduce the luck parameter (of enemies), or to increase it (of allies), depending on the djinn's upgrade. Titans or thunderbirds are better, of course, but colossi will do. A hero with the regeneration spell can overcome many obstacles with them. 4-5 of these units is already a formidable force...