A mage bomb is casting 1 cast of Blizzard (or Decay) on the enemy to make their units run towards your mage. You then cast Blizzard right in front of your mage. This will cause of all of those units who are coming to attack your mage, to sustain heavy damage. It works because enemy units that are not issued the stand ground command, will always run towards the mage that hits them with Blizzard. As such, mage bombing only works on grunts, axers, ogres, catapults, and theirequivalentt units. However, 90% of the time you will be doing mage bombs on ogres.
Just after this screenshot was taken, the spell lands on the ogres, and they start running towards the mage. You can see that the mage is not casting right now. He's about to start casting a new Blizzard just in front of him. Blizzard does not hurt the mage that casts it, so you have nothing to worry about from the spell itself. You also cannot cast the spell right on top of you. That is, if youpresss B for Blizzard, and then left click right on your mage, nothing will happen. Make sure you that cast 1 or 2 spaces off of your mage in the direction of the enemy.
The following shows the results of this mage bomb if the ogres are not moved out of harms way.
The result is a bunch of dead ogres. The remaining ogres all have under 15 hit points left. The mage was killed also, but it took 5 ogres with it, andseverelyy damaged 3 others. That is money very well spent. I bet with a few tries you could even do better than killing 5 ogres...a lot better.
If you prefer, you can cast the spell in front of your mage first. Cast it about 3 times, and then cast it once on the enemy. Then if you still have mana, switch back and cast it in front of your mage again. This way allows the spell to be 'built up' in front of your mage, causing more damage to the units when they enter into it. The problem with doing it this way is that good players will hear the spell being cast on your mage, and will simply run their ogres away. The other problem is that doing it this way costs more mana. You usually need 150+ on a mage to do it like this.
If you have multiple mages you can get really creative. One mage can hit the ogres to draw them towards you. A second mage can unleash in the open space between your first mage and where the ogres started from. The first mage casts just in front of him, and you've created a whole path of Blizzard which will damage the units for the entire trip that it takes them to get to your first mage. If you practise this a little bit, you will see how effective it is, and you'll even be able to kill 9 idle ogres with 2 mages, without losing either one them.
Everything you've read above applies to Death Knights also, although death and decay does not do as much damage to units. However, when you have 2 death knights, and cast haste on 1 of them, you now have a unit that is much more dangerous than a mage with Blizzard. Haste makes a death knight a lot faster... a LOT faster! Their walking speed increases tremendously, but it's their casting speed that is unbelievable.
Mage bombing isn't the same with hasted death knights. This is because death knights cast so fast, that you usually can't get them just to cast once on the enemy. Especially on higher game speeds, when the latency has been turned up, it becomes very hard to effectively do a mage bomb from across the screen with a hasted death knight. Not to worry though, for haste is fast enough to do it a different way. You can see how I've told my death knight to run just a few squares away from the enemy. Once the death knight gets in that position, you just cast decay on yourself, and chaos wilensueue.
When you have a hasted death knight, you can simply run close to the enemy units and cast in between you and them. Decay casts so fast that you will get at least 6 casts off before your death knight dies. It is very fast and will kill many units, even against the best players.
The great thing about this is that you can do this without having the area scouted. Death knights have a very long range of sight, and when they can move this fast, they can scout anything out for themselves. Even if you don't find many enemy units around, hasted death and decay takes out farms like nothing, and can kill all of the enemy peons mining gold in seconds.
When Enemies Attack...
When your death knights are attacked, it is usually good to cast a coil first, and then decay. If you just try to cast a death and decay, your death knight will die before he can get very much off, but if you cast the coil first, the dk's hit points will back up to full (you stole hp from an ogre or knight), which gives you more hit points to work with while casting.