Necropolis – “Misfortune never comes alone” [Campaign 2 – Mission 1]

Hello everyone.
I'm playing the first mission on a difficult level, and everything was going smoothly – I captured the last castle and was already relaxing, waiting for the second mission. And then comes the very last battle – to save Johans in prison by defeating a bunch of Luminaries. And there are so many of them, they attack without giving you a chance to retaliate – it's just awful.
How did you manage this battle?
Or should I wait a couple of months? But theoretically, the size of the enemy armies also increases over time.