Necromancer – “Invasion” [Campaign 3, Mission 3]

I'm currently playing through this mission... ugh... I barely managed to capture the last garrison. This garrison had 25 Titans, and it was labeled as "low danger," but I was initially scared. However, I then killed the Goblins, Mages, and Titans first, while the others stood there, stunned (it's a good spell, though). Then, I killed them one by one, resulting in not catastrophic losses, but I couldn't save about 1,000 Archers, and only about 500 remained :(
While Markel was gathering all of this, a strong Mage with high danger attacked Elizabeth, and she had to flee the city. In general, we captured two starting cities, and the rest are still with Markel. Now, I need to get 20 Dragons and recapture Elizabeth's city, and then we'll see what happens. In short, I'm having a hard time with this mission; cities are constantly being stolen, and we're being attacked, although I quickly destroyed the Elves. I'll keep playing.