Walkthrough tips, Discussion of Campaigns [Heroes 5]

Stuck on one stage... Necromancer - Invasion... on the stage where you need to take the cloak, staff, and ring from the mages... I captured two cities... but enemies still come and destroy everything, both in the city and outside... Moreover, I captured the city of Ziad with a Queen, and she didn't get the Staff of the Underworld!!! What should I do?? I brought a Necromancer to Ziad, and he didn't get the staff either, but the mages are still overwhelming me...
Help!!!
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