Necromancer – “Ruler of Erathia” [Campaign 3, Mission 5]

and is there also a time limit? I wasn't told anything. I remembered about my army: I had 12 Death Knights, which joined, 6 vampires, a small number of zombies, and about 150 skeleton archers. While all the infantry was busy with the phoenix, the archers were taking out the Death Knights, and I was successfully resurrecting them. Also, the Rakshasa's suggestion helped.