Necromancers – “Assault” [Campaign 3, Mission 2]

In the third mission of this campaign, you can acquire anything from the schools of Darkness and Summoning. It's just a matter of time: you need to build mage guilds in the captured cities until you learn how to summon a phoenix, celestial shield (which helped me a lot), and Darkness magic.

To do this, you need to try to already have a skilled Summoning skill.