This mission isn't as difficult as it seems. The key to playing as a Necromancer is not to rush. Patience is essential. You need to kill every living creature to replenish your army. Don't rush to leave the starting island; build up the Necromancer's city as much as possible so you don't set out with an empty army. After that, it gets easier. Personally, I always capture Ziyad first. And there are also the Mountains of Rot nearby to the right, which are eager to join you. The most challenging part is dealing with the Elves. You'll have to put in some effort with Isabel. After defeating the Elf, the easiest part of the mission remains. Simply have Markel capture the middle city, and leave Isabel on the far left. After that, I have a decent army. I approached Cyrus with confidence. Liches guard the garrisons because they are clustered there. And that's the end of the mission, except for the battle with Mahir and the dragons. It's important to develop Markel correctly. I use two different development paths, both of which are good.
1. The Path of Chaos. Take the basics of Chaos (cold damage, chilling aura). Also, take spell power. Choose undead protection from magic and secret advantage. From education, take scholar of the dead and, most importantly, magic attraction, because the Necromancer's mana pool is small. You can also take luck for support.
2. The Path of Darkness. This path focuses on troops. Develop education with power of the dead and secret revelation. Magic of darkness: cursed earth. You can also take power of pain. Then, develop attack with cold steel and general acceleration, and defense with grave chill. Good luck, and if you have questions about specific parts of the walkthrough, feel free to message me privately.