I like this mission. It's pretty unique. It's like a race between you and Mal'Ganis to get to the zombies. Knights are available for the first time which is very cool as you finally have something that can stand a chance against an abomination. Although you probably won't experience many attacks from abominations on this level you will be attacked by Mal'Ganis. He's pretty powerful and can put units to sleep. If he puts a unit to sleep simply force another unit to attack the sleeping one and it will be awakened, although this is only really worth doing if the sleeping unit is Arthas or a knight.

To begin, build up your base. Don't go after the zombies right away. You can build up a few attack forces and can even send one only to attack Mal'Ganis. Take Arthas with you to defeat Mal'Ganis (holy light will work wonders here) and build about six footmen to go out and destroy plagued houses - the northwest corner is a very target rich environment. Go for another attack group and send them to a different part of the city. You'll have about three minutes before Mal'Ganis comes back so build up your base and try to destroy as many plagued houses as possible. With Arthas and two groups of footmen working to destroy as many houses as possible (don't let the men sit idle too long!) you shouldn't have any trouble beating Mal'Ganis. I did have to take him out about two more times before I finished this level, but Mal'Ganis didn't break 40 zombies so I was happy with the outcome. Don't neglect your base defenses as you will be attacked by some powerful groups as you get near 100. If you're close to your goal of 100 and an enemy group is attacking your base finish up the 100 and thus, the mission if you can, before they destroy the base.