I though of a Mob that shows his health to use it as a tank, i had 2 choices, the enderdragon and the wither, and obviously the enderdragon is a bad choice, so i chose the wither.
Later, i made him ride an invisible invulnerable zombie following a villager to make his defined path, but i couldn't get him to ride the zombie using an Mcedit filter, i thought it was something wrong with MCedit, so i switched to NBTedit and created the Tag "Riding "with all the required options for an invulnerable zombie, and made a spawner for him.
when i spawned him though, it looked to me as if he was riding the zombie, so i tried to shoot him, but apparently he was only above the zombie, not riding him, i tried spawning another one and the same happened.
Mojang please fix this, i highly depend on Mojang to fix this so my map works .
EDIT: this might seem like a Duplicate but it is not, the other guy tried getting him into minecarts, which is something mojang removed to prevent abusing the wither by spawning him into a minecart and killing him inside it. the code however, prevents him from riding ANY entity even through NBTediting and third party programs.