Summoned Ender Dragon is unkillable

When I summon the Ender Dragon, it hovers in one spot and I can't kill it. Whenever I do this, I would have to abandon my worlds forever, even when it's my most favorite world. :..( This happens in every version of Minecraft, even 1.10.2. Can you please fix this so that the summoned Ender Dragon is killable? Because I don't want to permanently abandon my worlds.