Yes, it's a bit harder at the beginning. But only if the goal is to complete the game without any losses. If losses are acceptable, there are no problems at all. And even without losses, it's enough to find a decent, powerful spell, or a spell that provides troop resurrection, or self-healing troops (the latter is preferable), and everything becomes very easy.
Anticipating disbelief and attempts to prove that I am wrong, I will say that I completed the game with a mage without any losses on the impossible difficulty.
In terms of troops, I used droids (with Bolo) in the early stages, then two types of vampires (from scrolls), and sometimes trolls (also from scrolls), and in the end - black and red dragons (also from scrolls). I didn't buy troops; I multiplied them through sacrifice. Instead of droids and vampires, I think paladins would work well (especially if you have Phantom). I didn't use demonologists or Incas, although they are probably also good.
I had practically all the spells, but I used these most actively: oil cloud, fire storm, fire bolt (the most effective spell in terms of price/quality ratio), sacrifice. Less often - geyser, time warp, hypnosis, phantom, resurrection. The rest, I think, were only used in specific cases and to level up medals. By the way, I leveled up all the medals.
By the way, I learned a lot of useful things for a successful playthrough on the official game forum: http://kingsbounty.ru/forum/index.php?showforum=25.