Spawn a fake wither boss with ProtocolLib 3.0.0+ and Minecraft 1.7.2 (though earlier versions are supported).

SpawnFakeWither.java

package com.comphenix.example;

import org.bukkit.command.Command;
import org.bukkit.command.CommandSender;
import org.bukkit.plugin.java.JavaPlugin;

import com.comphenix.protocol.ProtocolLibrary;
import com.comphenix.protocol.events.PacketContainer;
import java.lang.reflect.InvocationTargetException;
import java.util.logging.Level;

import org.bukkit.Bukkit;
import org.bukkit.Location;
import org.bukkit.entity.EntityType;
import org.bukkit.entity.Player;
import org.bukkit.scheduler.BukkitTask;

import com.comphenix.protocol.ProtocolManager;
import com.comphenix.protocol.wrappers.WrappedDataWatcher;

public class SpawnFakeWither extends JavaPlugin {
    private static final int TICKS_PER_SECOND = 20;
    
    // You could also use a full-fledged API like RemoteEntities
    private static class FakeWither {
        public static final byte INVISIBLE = 0x20;
        // Just a guess
        private static final int HEALTH_RANGE = 80 * 80;
        // Next entity ID
        private static int NEXT_ID = 6000;
        
        private static final int METADATA_WITHER_HEALTH = 6; // 1.5.2 -> Change to 16
        
        // Metadata indices
        private static final int METADATA_FLAGS = 0;
        private static final int METADATA_NAME = 10;        // 1.5.2 -> Change to 5
        private static final int METADATA_SHOW_NAME = 11;   // 1.5.2 -> Change to 6
        
        // Unique ID
        private int id = NEXT_ID++;
        // Default health
        private int health = 300; 
        
        private boolean visible;
        private String customName;
        private boolean created;
        
        private Location location;
        private ProtocolManager manager;

        public FakeWither(Location location, ProtocolManager manager) {
            this.location = location;
            this.manager = manager;
        }

        public int getHealth() {
            return health;
        }
        
        public void setHealth(int health) {
            // Update the health of the entity
            if (created) {
                WrappedDataWatcher watcher = new WrappedDataWatcher();
                
                watcher.setObject(METADATA_WITHER_HEALTH, (float) health); // 1.5.2 -> Change to (int)
                sendMetadata(watcher);
            }
            this.health = health;
        }
        
        public void setVisible(boolean visible) {
            // Make visible or invisible
            if (created) {
                WrappedDataWatcher watcher = new WrappedDataWatcher();
                
                watcher.setObject(METADATA_FLAGS, visible ? (byte)0 : INVISIBLE);
                sendMetadata(watcher);
            }
            this.visible = visible;
        }
        
        public void setCustomName(String name) {
            if (created) {
                WrappedDataWatcher watcher = new WrappedDataWatcher();
                
                if (name != null) {
                    watcher.setObject(METADATA_NAME, name);
                    watcher.setObject(METADATA_SHOW_NAME, (byte) 1);
                } else {
                    // Hide custom name
                    watcher.setObject(METADATA_SHOW_NAME, (byte) 0);
                }
                
                // Only players nearby when this is sent will see this name
                sendMetadata(watcher);
            }
            this.customName = name;
        }
        
        private void sendMetadata(WrappedDataWatcher watcher) {
            WrapperPlayServerEntityMetadata update = new WrapperPlayServerEntityMetadata();
            
            update.setEntityId(id);
            update.setEntityMetadata(watcher.getWatchableObjects());
            broadcastPacket(update.getHandle(), true);
        }
        
        public int getId() {
            return id;
        }
        
        public void create() {
            WrapperPlayServerSpawnEntityLiving spawnMob = new WrapperPlayServerSpawnEntityLiving();
            WrappedDataWatcher watcher = new WrappedDataWatcher();
            
            watcher.setObject(METADATA_FLAGS, visible ? (byte)0 : INVISIBLE);
            watcher.setObject(METADATA_WITHER_HEALTH, (float) health); // 1.5.2 -> Change to (int)
            
            if (customName != null) {
                watcher.setObject(METADATA_NAME, customName);
                watcher.setObject(METADATA_SHOW_NAME, (byte) 1);
            }
            
            spawnMob.setEntityID(id);
            spawnMob.setType(EntityType.WITHER);
            spawnMob.setX(location.getX());
            spawnMob.setY(location.getY());
            spawnMob.setZ(location.getZ());
            spawnMob.setMetadata(watcher);
            
            broadcastPacket(spawnMob.getHandle(), true);
            created = true;
        }
        
        public void destroy() {
            if (!created)
                throw new IllegalStateException("Cannot kill a killed entity.");
            
            WrapperPlayServerEntityDestroy destroyMe = new WrapperPlayServerEntityDestroy();
            destroyMe.setEntities(new int[] { id });
            
            broadcastPacket(destroyMe.getHandle(), false);
            created = false;
        }
        
        private void broadcastPacket(PacketContainer packet, boolean onlyNearby) {
            for (Player player : Bukkit.getServer().getOnlinePlayers()) {
                // Must be within the range
                if (!onlyNearby || player.getLocation().distanceSquared(location) < HEALTH_RANGE) {
                    try {
                        manager.sendServerPacket(player, packet);
                    } catch (InvocationTargetException e) {
                        Bukkit.getLogger().log(Level.WARNING, "Cannot send " + packet + " to " + player, e);
                    }
                }
            }
        }
    }
    
    private FakeWither wither;
    private BukkitTask task;
    
    @Override
    public boolean onCommand(CommandSender sender, Command command, String label, String[] args) {
        if (sender instanceof Player) {
            Player player = (Player) sender;
            
            if (wither != null) {
                wither.destroy();
                task.cancel();
            }
            
            // Initialize and create the wither
            wither = new FakeWither(player.getLocation(), ProtocolLibrary.getProtocolManager());
            wither.setCustomName("Countdown Man");
            wither.setVisible(false);
            wither.create();

            // Count down
            task = getServer().getScheduler().runTaskTimer(this, new Runnable() {
                @Override
                public void run() {
                    // Count down
                    wither.setHealth(wither.getHealth() - 1);
                    
                    if (wither.getHealth() <= 0) {
                        wither.destroy();
                        task.cancel();
                    }
                }
            }, TICKS_PER_SECOND / 4, TICKS_PER_SECOND / 4);
        }
        return true;
    }   
}

Also add AbstractPacket, WrapperPlayServerEntityDestroy, WrapperPlayServerEntityMetadata and WrapperPlayServerSpawnEntityLiving from PacketWrapper.