This is the easiest way to beat any of the 3 island bosses not only the dragon. First you need Deinonychus as many as you can bring next you need to have a rex to ride. So if you doing solo bring 1 rex and 19 deinonychus (20 tame limit)

Then once in the boss fight run around on your rex. Have the deinonychus follow you as soon as the dragon lands get some distance and whistle your deinonychus to attack the dragon. When the deinonychus run into the dragon they will block it and the dragon will not attack them. The dragon will continually walk trying to get to the rex as long as you keep your distance with the rex and have the deinonychus attacking you will take no damage and the dragon will be stuck and won’t attack. Essentially the bosses all have the same mechanics they will always attack the biggest Dino in the arena (biggest by size not level) so this essentially can be done with a rex and any Dino smaller I just use deinonychus and recommend them because of their bleed effect. And they are easy to raise