Class that allows you to display text using the boss health bar. Note: Whenever you see "MinecraftFPS.getInstance()", replace that with the instance of your main class.

Utilizing the Boss Health Bar

package com.chasechocolate.example;

import java.lang.reflect.Field;
import java.util.HashMap;

import net.minecraft.server.v1_6_R2.DataWatcher;
import net.minecraft.server.v1_6_R2.EntityPlayer;
import net.minecraft.server.v1_6_R2.Packet;
import net.minecraft.server.v1_6_R2.Packet205ClientCommand;
import net.minecraft.server.v1_6_R2.Packet24MobSpawn;
import net.minecraft.server.v1_6_R2.Packet29DestroyEntity;
import net.minecraft.server.v1_6_R2.Packet40EntityMetadata;

import org.bukkit.Location;
import org.bukkit.craftbukkit.v1_6_R2.entity.CraftPlayer;
import org.bukkit.entity.EntityType;
import org.bukkit.entity.Player;
import org.bukkit.scheduler.BukkitRunnable;

public class PacketUtils {
  public static final int ENTITY_ID = 1234;
	
	private static HashMap<String, Boolean> hasHealthBar = new HashMap<String, Boolean>();
	
	public static void sendPacket(Player player, Packet packet){
		EntityPlayer entityPlayer = ((CraftPlayer) player).getHandle();
		
		entityPlayer.playerConnection.sendPacket(packet);
	}
	
	//Accessing packets
	public static Packet24MobSpawn getMobPacket(String text, Location loc){
		Packet24MobSpawn mobPacket = new Packet24MobSpawn();
		
		mobPacket.a = (int) ENTITY_ID; //Entity ID
		mobPacket.b = (byte) EntityType.WITHER.getTypeId(); //Mob type (ID: 64)
		mobPacket.c = (int) Math.floor(loc.getBlockX() * 32.0D); //X position
		mobPacket.d = (int) Math.floor(loc.getBlockY() * 32.0D); //Y position
		mobPacket.e = (int) Math.floor(loc.getBlockZ() * 32.0D); //Z position
		mobPacket.f = (byte) 0; //Pitch
		mobPacket.g = (byte) 0; //Head Pitch
		mobPacket.h = (byte) 0; //Yaw
		mobPacket.i = (short) 0; //X velocity
		mobPacket.j = (short) 0; //Y velocity
		mobPacket.k = (short) 0; //Z velocity
		
		DataWatcher watcher = getWatcher(text, 300);
		
		try{
			Field t = Packet24MobSpawn.class.getDeclaredField("t");
			
			t.setAccessible(true);
			t.set(mobPacket, watcher);
		} catch(Exception e){
			e.printStackTrace();
		}
		
		return mobPacket;
	}
	
	public static Packet29DestroyEntity getDestroyEntityPacket(){
		Packet29DestroyEntity packet = new Packet29DestroyEntity();
		
		packet.a = new int[]{ENTITY_ID};
		
		return packet;
	}
	
	public static Packet40EntityMetadata getMetadataPacket(DataWatcher watcher){
		Packet40EntityMetadata metaPacket = new Packet40EntityMetadata();
		
		metaPacket.a = (int) ENTITY_ID;
		
		try{
			Field b = Packet40EntityMetadata.class.getDeclaredField("b");
			
			b.setAccessible(true);
			b.set(metaPacket, watcher.c());
		} catch(Exception e){
			e.printStackTrace();
		}
		
		return metaPacket;
	}
		
	public static Packet205ClientCommand getRespawnPacket(){
		Packet205ClientCommand packet = new Packet205ClientCommand();
		
		packet.a = (int) 1;
		
		return packet;
	}
	
	public static DataWatcher getWatcher(String text, int health){
		DataWatcher watcher = new DataWatcher();
		
		watcher.a(0, (Byte) (byte) 0x20); //Flags, 0x20 = invisible
		watcher.a(6, (Float) (float) health);
		watcher.a(10, (String) text); //Entity name
		watcher.a(11, (Byte) (byte) 1); //Show name, 1 = show, 0 = don't show
		//watcher.a(16, (Integer) (int) health); //Wither health, 300 = full health
		
		return watcher;
	}
	
	//Other methods
	public static void displayTextBar(String text, final Player player){
		Packet24MobSpawn mobPacket = getMobPacket(text, player.getLocation());
		
		sendPacket(player, mobPacket);
		hasHealthBar.put(player.getName(), true);
		
		new BukkitRunnable(){
			@Override
			public void run(){
				Packet29DestroyEntity destroyEntityPacket = getDestroyEntityPacket();
				
				sendPacket(player, destroyEntityPacket);
				hasHealthBar.put(player.getName(), false);
			}
		}.runTaskLater(MinecraftFPS.getInstance(), 120L);
	}
	
	public static void displayLoadingBar(final String text, final String completeText, final Player player, final int healthAdd, final long delay, final boolean loadUp){
		Packet24MobSpawn mobPacket = getMobPacket(text, player.getLocation());
		
		sendPacket(player, mobPacket);
		hasHealthBar.put(player.getName(), true);
		
		new BukkitRunnable(){
			int health = (loadUp ? 0 : 300);
			
			@Override
			public void run(){
				if((loadUp ? health < 300 : health > 0)){
					DataWatcher watcher = getWatcher(text, health);
					Packet40EntityMetadata metaPacket = getMetadataPacket(watcher);
					
					sendPacket(player, metaPacket);
					
					if(loadUp){
						health += healthAdd;
					} else {
						health -= healthAdd;
					}
				} else {
					DataWatcher watcher = getWatcher(text, (loadUp ? 300 : 0));
					Packet40EntityMetadata metaPacket = getMetadataPacket(watcher);
					Packet29DestroyEntity destroyEntityPacket = getDestroyEntityPacket();
					
					sendPacket(player, metaPacket);					
					sendPacket(player, destroyEntityPacket);
					hasHealthBar.put(player.getName(), false);
					
					//Complete text
					Packet24MobSpawn mobPacket = getMobPacket(completeText, player.getLocation());
					
					sendPacket(player, mobPacket);
					hasHealthBar.put(player.getName(), true);
					
					DataWatcher watcher2 = getWatcher(completeText, 300);
					Packet40EntityMetadata metaPacket2 = getMetadataPacket(watcher2);
					
					sendPacket(player, metaPacket2);
					
					new BukkitRunnable(){
						@Override
						public void run(){
							Packet29DestroyEntity destroyEntityPacket = getDestroyEntityPacket();
							
							sendPacket(player, destroyEntityPacket);
							hasHealthBar.put(player.getName(), false);
						}
					}.runTaskLater(MinecraftFPS.getInstance(), 40L);
					
					this.cancel();
				}
			}
		}.runTaskTimer(MinecraftFPS.getInstance(), delay, delay);
	}
	
	public static void displayLoadingBar(final String text, final String completeText, final Player player, final int secondsDelay, final boolean loadUp){
		final int healthChangePerSecond = 300 / secondsDelay;
		
		displayLoadingBar(text, completeText, player, healthChangePerSecond, 20L, loadUp);
	}
}

Thanks.