It's interesting, with such an army, on what day or month did you play?
Regarding mana, there's a loophole. You are released with mana when you pass into the last and main castle. You can stand there for a couple of months and restore your mana, or you can do it another way. At the top, there are buildings for recruiting units (black dragons), and the land is cursed, so the teleport spell won't work. Therefore, before the portal (on normal land), cast the "fly" spell and enter the portal. After teleporting (the spell is still active), fly to the castle and attack. As a result, you have a lot of mana, the hero's luck is high, and the morale is not low.
It worked for me... Good luck.