As you all know that Death Knights can't be hit by their own Blizzard or Death and Decay! If someone attacks your Death Knight, cast Death and Decay right in front of you and walk into it or have the Death Knight cast it on itself. The enemy must walk through the Death and Decay to reach the Death Knight. If you want to kill a group of Peasants, stand in the middle of them then cast a Blizzard right on top of the Mage.
Blizzards and Death and Decay increase in intensity and damage the longer they go. If you want to do the maximum amount of damage, cast it once and let it continue until out of mana.
First thing first, you can take out a huge army of Ogres or Knights with only 2 Death Knights. Cast Death & Decay right in front of your Death Knight. Wait till the Death & Decay is going for a few seconds then use the other Death Knight to cast Death and Decay right on the Ogres or Knights. The Ogres will then charge into the already going Death and Decay and will be severely damaged and sometimes all be killed.
Then you have the abilities to ake the Death Knight and cast just to the side of a Tower rather than clicking on the building. Then cast the Blizzard. Watch the mana bar. When you see 50 mana - 75 mana go down on the mana meter, hit stop on the Mage. You can also watch the Death Knight or Mage's arms. When you see them move 2-3 times, stop.
Depending on how you aim the Blizzard sometimes you can destroy Guard Towers with only two casts of Blizzard while sometimes times it takes 3. Cannon Towers seem to take at least 3 casts to kill.
But how to make a Mage Invisible and walk all the way to your enemy's mine? The shortcut is to cast a Blizzard on the enemy Peons. It's best to stand in the middle of the Peons and cast the Blizzard as close to you as possible. Aim the Blizzard aiming icon as close to the mage as the computer will allow. Then cast the blizzard in a line from their Great Hall to the Mine. You don't have to worry about hitting yourself. It is almost impossible to kill that Mage with the Peons. We tried it.
You don't have to be in the middle of the Peons. You can cast Blizzard on the Peons from a distance away if the Peons are heavily defended.
The only Orc equivalent is to sneak near the mine and try Death and Decay. You could cast Unholy Armor and Haste on the Death Knight and Walk straight to the mine. But this really does not compare to ease of Invisibility.
For most of Death Knights, one of the best defenses is to wall of the town entirely so no enemy unit can get in. This can be hard to do while making your town so you can get out but using Barracks it is possible.
Another is to keep a Mage near your mine and cast Polymorph on any enemy mage that starts to Blizzard. If you are Orc, use Death Coil. You can put a few arrows or cannon Towers close to your mine. You can place a few Archers or Axe Throwers near the mine to kill the Mage and still be out of range of the Blizzard. The whole key is to have these guys a little away from the mine so they will not get hit by the Blizzard. Some Cannon Towers and some Archers probably would be the best defense.