Gray Alliance

Interesting campaign.

I'm struggling to get through the 3rd mission.
How did you guys manage it? I have a feeling I'm going to spend a couple of in-game months on this, and then an opponent will show up with a thousand-strong army and wipe me out. Not only are there 3 cities against one (and it's not easy to capture the second Academy and the Human city), but this useless mage is stubbornly leveling up his Knowledge instead of normal attributes.
Ugh, I hate classes that focus on Knowledge.
I don't know how they're supposed to fight. He won't be able to do anything.

Don't worry, the enemy is just chilling in their castles here. Only the mission boss is tough, but you only encounter him at the very end, after you've gathered a large enough army. Before that, there's a mentor. I recommend taking the Mentorship skill for the 4th mission (you can re-spec it later).
Also, in this mission, King Alex from the 1st mission joins you; I recommend having him visit all the upgrade locations and also giving him the Mentorship skill.
And so that Cyrus can get through all of this, it's best to farm some decent levels for him (yes, it's inconvenient, the mission starts with a fight).