First you take two Death Knights/Mages with full mana and Death and Decay or Blizzard. Take up position a bit back from a group of enemies.

With the first Death Knight, cast Death and Decay just in front of your Death Knights. Let the Death Knight continue to cast until he is out of mana.

Immediately after the first Death Knight starts casting, command the second Death Knight to cast one cast of Death and Decay on the enemy: hit D for Death and Decay, click on the enemy then Immediately hit Stop so the Death Knight will only use one cast. Then use the remaining mana to cast another Death and Decay right in front of the two Death Knights.

What will happen is the first Death Knight casts and gets a nice heavy rain of Death and Decay going. The second Death Knight uses one Death and Decay cast to "wake up" the enemy. The charge the Death Knights and run into a fully damaging Death and Decay area. Many if not all of the enemy units are killed. This doesn't always happen so it's a good idea, if possible, to have some troops nearby to clean up survivors. Some Archers would help in this situation but most likely, you will be using Paladins or Ogres.

You can do the same thing on water maps. Just pop out of a transport do a Double Blizzard then get back in the Transport and run!