Hello Josh014, I don't think you can, because its client side. :3 For Further reading: https://forums.bukkit.org/threads/boss-health-bar.135823/ http://forums.bukkit.org/threads/creating-a-virtual-boss-health-bar.120821/ http://dev.bukkit.org/bukkit-plugins/health-bar/
I saw a server (Kit pvp server) with a boss health bar with the player name on top of it when he reached the 500 kill streak.
really? wow thats a quite cool feature try use the Packet0x18MobSpawn :3 http://forums.bukkit.org/threads/using-the-boss-health-bar.99607/
Hmm thank you but not really what I search because the boss health bar is the players health bar so on a 500 kill streak you get 300 health like a wither boss health bar.
That's easy. Spawn in a wither and make it invisible. Create a repeating task that keeps the wither from moving. Make sure it doesn't shoot too. Change it's name to the players name and create an equation that sets the bar to the same amount of health as the player.