You don't need diamond armor. Any armor at all prevents damage and all that the dragon will do is push you around and even then any time you hit it, it will turn around and fly away until its next approach. The dragon is easy to kill but it takes a lot of hits to do it. Do not waste time and energy getting diamond armor though especially since it can happen that you get pushed off the edge of the island and then your diamond armor is gone forever. Totally unnecessary.
What level of diffuculty is your experience based on?
I'm currently gearing up to go for it on hard.