1. Neglecting troops and city development. A small group of Archers and a Mage cannot win, and that's a fact – especially if the enemy hero is Nasir. As a result, in the final battle, the Tower faction's army is quickly defeated by the Academy, while the Necromancer doesn't have time to eliminate the enemy army.
In general, it's quite amusing to watch a battle between two heroes who have focused on magic – the armies line up against each other and wait for the heroes to exchange magical attacks. :)
Therefore, the Necromancer should not forget about combat skills (Leadership, Luck – which may be useful for the Lucky Charm spell, Logistics, etc.).
2. In my opinion, a very serious mistake is to stay locked inside the city when the Mage comes to visit. The Academy's troops are focused on ranged and magical attacks, so you need to catch him in an open field, rather than waiting for your own walls to hinder access to his units.
Naturally, the League's troops huddled inside the castle walls and their small numbers are simply a gift for Nasir.
Wüst und leer und ohne Segen.
Hier bin ich durch dich gefangen,
Muss für dich durchs Feuer gehen.
Du siehst mich in Flammen stehen,
Glühend will ich dich empfangen.