I went in The End with full enchanted diamond armor with the highest protection levels possible along with a awesome diamond sword and a Infinity + Power bow.(Forgot what level it is) I even took around 5 stacks of snowballs in and a ton of potions just in case he was close but... All it took was a long time to shoot all the totems down that heal him and with enough aiming, the dragon was downed easily.
In my opinion the dragon needs to be more powerful rather then just completely relying then the healing stuff that you need to shoot down... Once those are down he's as weak as a zombie anyways, provided you have enough arrows and that your not stupid enough to look at an enderman.
Try fighting a Wither alone, that's way more harder even if you have good armor...(The Wither effect + explosions can really mess you up.