Minecraft Forums

Ok. First off, for this tutorial, and most of my tutorials, you will need an NBT editing program, that can handle edits to the map files. NBTEdit is NOT capable if this. For this, I highly recommend NBTExplorer, which is much more useful.
So, requirements:
Minecraft
NBTExplorer or equivalent (NOT NBTEdit!)

And now the tutorial.
To start off, you're going to need a map already created and used in minecraft or mcedit. This is because you will need access to the mob you want to edit. After the map is created, place down an NPC Spawner of your chosen mob you want to have custom health. This will not make them spawn with this health. The Spawner is just a place maker. If you're chosen mob is a mob that can be generated into the world, I recommend placing down a tileEntity near the spawner on the same chunk. tileEntitys (Yes, the proper spelling of those is like that, as it is a coded property that is unique, and thus a name, so it would be spelled exactly as in the code) are blocks that do something besides sit there as decoration. Examples would be a furnace, chest, falling gravel (Not gravel that is just sitting there), lit TNT, fire, and falling sand. Things that just give off light, like torches, are not tile entities.

Once you have this done, exit minecraft and open up NBTExplorer.
Navigate to the folder you're map is in, making sure to use the open folder button, rather than the open file button. Once there, click the find button, and type in the name of you're mob. If you misspelled it, click on the find button in the menu at the top, as the button defaults to finding the next one, rather than starting a new search.

Look for the mob spawner in the tile entities list and the land marks that you placed down earlier in the tileEntitys list. When you find it, navigate to one of the mobs in that chunk, and change their health property to something you want it to be. The game automatically adjusts to fit higher mob healths.