cant kill enderdragon

when my friend and I were playing in a world and we went to the end i tryed to kill the enderdragon but it wont take meelee damage. even the /kill command did not worked when i typed /kill @e[type=ender_dragon] it said no targets match. i even typed /kill @e and it still did not work. it only took a little bit of damage by beds. plz fix this bug.