Dragon won't die

If you summon the dragon in the overworld, even if you have it constantly being teleported to 0,0, the dragon won't die, and when you stop the teleporting, it doesn't die, you have to kill it again. (Note: I'm sure, because it should easily die to a power 5000 bow in atleast a couple of hits, and even then I hit it with a power 5 sword about 100 times, and still, nothing)