Phew that was hard! I beat the dragon in the end.
Cool boss rush game - I see that you're using gamemaker - if you'd like to do a web export for itch so people can play in browser check out https://www.reddit.com/r/gamemaker/comments/1lilw93/gx_games_export_target_on_it...
Nice job making all of the bosses have a different challenge. At first I thought each one was too powerful, but I eventually figured out ways to beat each of them without losing health