Ender Dragon is bugged

I visited The End to cerate basen on End City, but The dragon is bugged. When I come to End, I saw ender portal to Overworld active. Dragon can't been hit, but its moved normally. When I use /kill @e[type=ender_dragon], it didn't work. Wiem I used /kill @e, it killed only Endermen. I'm planning to actually kill Ender Dragon, and with this bug I can't!

Sorry for my english: I'm Polish, and I'm wirting this on phone.