Minecraft Speedrunning/Ender Dragon Fight
You will need to have gathered some resources at some point throughout the run. Minimal resources are some beds which you can use to "one cycle" the dragon. This works since beds explode when you try to sleep with them in the end. This means when the dragon flies to the middle, you place a bed on the bedrock and explode it at the right time, dealing damage to the dragon and pushing it up so you can use another bed to also deal damage and push up the dragon again. This can take a lot of practice and the best runners can consistently kill the dragon in 4-5 beds.
Probably the easiest way to kill the dragon is to have a bow and at least 32 arrows. Then you can shoot arrows at the end crystals to take them down while making sure to not look at the endermen and move away when the dragon shoots dragon breath at you. When the dragon perches you can move around to the side or behind the dragon to enter the fountain underneath the dragon and hit it with an axe or sword until it flies away. If you are not in the fountain when it flies away from the fountain be careful since it will fly towards you, potentially knocking you up into the air. As long as you've shot down every crystal the dragon won't heal. Note that when shooting at the dragon or hitting it with an axe/sword hitting its head does much more damage than hitting its body.
After killing the dragon and you enter the end portal, timing ends and you've completed a speedrun!
There are still many, many advanced, complex and varied strategies left unmentioned here. Check out other guides, resources, streams, runs and discord servers for information on these.