"Escape to Salvation" - "Magnificent Retreat" [Campaign 4, Mission 4]

You put the Thunderbirds aside, and remove all the others. You control one of the attack stacks of Deleb, cover the Thunderbirds with it, and cast Vampirism on them. Then, you destroy the enemy archers and the ballista (!). Try to ensure that only the enemy unit you control remains. But many Thunderbirds will die. And "Vampirism" won't last long, and with it, "Resurrection" won't work. Then there are two options: A) "Antimagic", and then "Resurrection" until the Titans are fully restored. B) "Raise Dead". But no Antimagic! The "Raised" Thunderbirds will remain, as under Vampirism they acquire all the properties of the undead, including "survival" after a battle with "Raise Dead". In general, as long as you have enough mana. And you need at least 30 Thunderbirds. By the way, you don't necessarily have to defeat Deleb. You can run past her so that she doesn't chase you and block the portal. I didn't even notice her during my first playthrough :o.