Attacking hidden players. Version for 1.7.2: https://gist.github.com/aadnk/d4f5d49fe079cb13da9c

AttackingHiddenPlayers.java

package com.comphenix.example;

/*
* Attack hidden players
*
* Copyright 2012 Kristian S. Stangeland (Comphenix)
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library. If not, see <http://www.gnu.org/licenses/>.
*/

import java.util.Random;

import org.bukkit.ChatColor;
import org.bukkit.Location;
import org.bukkit.command.Command;
import org.bukkit.command.CommandSender;
import org.bukkit.entity.Player;
import org.bukkit.event.Listener;
import org.bukkit.plugin.PluginManager;
import org.bukkit.plugin.java.JavaPlugin;

import com.comphenix.protocol.Packets;
import com.comphenix.protocol.ProtocolLibrary;
import com.comphenix.protocol.ProtocolManager;
import com.comphenix.protocol.events.ConnectionSide;
import com.comphenix.protocol.events.PacketAdapter;
import com.comphenix.protocol.events.PacketContainer;
import com.comphenix.protocol.events.PacketEvent;
 
public class AttackingHiddenPlayers extends JavaPlugin implements Listener {
    private ProtocolManager protocolManager;

    public void onLoad() {
        protocolManager = ProtocolLibrary.getProtocolManager();
    }
 
    public void onEnable() {        
        PluginManager pm = getServer().getPluginManager();
        pm.registerEvents(this, this);
        
        // This is where the magic happens
        protocolManager.getAsynchronousManager().registerAsyncHandler(
                new PacketAdapter(this, ConnectionSide.CLIENT_SIDE, Packets.Server.ARM_ANIMATION) {
                    
            @Override
            public void onPacketReceiving(PacketEvent event) {
                final int ATTACK_REACH = 4;
                Random rnd = new Random();
                
                Player observer = event.getPlayer();
                Location observerPos = observer.getEyeLocation();
                Vector3D observerDir = new Vector3D(observerPos.getDirection());
                
                Vector3D observerStart = new Vector3D(observerPos);
                Vector3D observerEnd = observerStart.add(observerDir.multiply(ATTACK_REACH));
                
                Player hit = null;
                
                // Get nearby entities
                for (Player target : protocolManager.getEntityTrackers(observer)) {
                    // No need to simulate an attack if the player is already visible
                    if (!observer.canSee(target)) {
                        // Bounding box of the given player
                        Vector3D targetPos = new Vector3D(target.getLocation());
                        Vector3D minimum = targetPos.add(-0.5, 0, -0.5); 
                        Vector3D maximum = targetPos.add(0.5, 1.67, 0.5); 
    
                        if (hasIntersection(observerStart, observerEnd, minimum, maximum)) {
                            if (hit == null || hit.getLocation().distanceSquared(observerPos) > target.getLocation().distanceSquared(observerPos)) {
                                hit = target;
                            }
                        }
                    }
                }
                
                // Simulate a hit against the closest player
                if (hit != null) {
                    PacketContainer useEntity = protocolManager.createPacket(Packets.Client.USE_ENTITY, false);
                    useEntity.getIntegers().
                        write(0, observer.getEntityId()).
                        write(1, hit.getEntityId()).
                        write(2, 1 /* LEFT_CLICK */);
                    
                    // Chance of breaking the visibility
                    if (rnd.nextDouble() < 0.3) {
                        toggleVisibilityNative(observer, hit);
                    }
                    
                    try {
                        protocolManager.recieveClientPacket(event.getPlayer(), useEntity);
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            }
            
            // Get entity trackers is not thread safe
        }).syncStart();
    }
    
    // Source:
    //    http://www.gamedev.net/topic/338987-aabb---line-segment-intersection-test/
    private boolean hasIntersection(Vector3D p1, Vector3D p2, Vector3D min, Vector3D max) {
        final double epsilon = 0.0001f;
        
        Vector3D d = p2.subtract(p1).multiply(0.5);
        Vector3D e = max.subtract(min).multiply(0.5);
        Vector3D c = p1.add(d).subtract(min.add(max).multiply(0.5));
        Vector3D ad = d.abs();

        if (Math.abs(c.x) > e.x + ad.x)
            return false;
        if (Math.abs(c.y) > e.y + ad.y)
            return false;
        if (Math.abs(c.z) > e.z + ad.z)
            return false;
      
        if (Math.abs(d.y * c.z - d.z * c.y) > e.y * ad.z + e.z * ad.y + epsilon)
            return false;
        if (Math.abs(d.z * c.x - d.x * c.z) > e.z * ad.x + e.x * ad.z + epsilon)
            return false;
        if (Math.abs(d.x * c.y - d.y * c.x) > e.x * ad.y + e.y * ad.x + epsilon)
            return false;
                
        return true;
    }
    
    private void toggleVisibilityNative(Player observer, Player target) {
        if (observer.canSee(target)) {
            observer.hidePlayer(target);
        } else {
            observer.showPlayer(target);
        }
    }
    
    @Override
    public boolean onCommand(CommandSender sender, Command command, String label, String[] args) {
        Player observer = null;
        Player target = null;
        
        // Get the target argument
        if (args.length > 0) {
            target = getServer().getPlayerExact(args[0]);
        } else {
            sender.sendMessage(ChatColor.RED + "This command requires at least one argument.");
            return true;
        }
        
        // Get the observer argument
        if (args.length == 2) {
            observer = getServer().getPlayerExact(args[1]);
        } else {
            if (sender instanceof Player) {
                observer = (Player) sender;
            } else {
                sender.sendMessage(ChatColor.RED + "Optional parameter is only valid for player commands.");
                return true;
            }
        }
        
        toggleVisibilityNative(observer, target);
        return true;
    }
}

Vector3D.java

package com.comphenix.example;

/*
* Attack hidden players
*
* Copyright 2012 Kristian S. Stangeland (Comphenix)
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library. If not, see <http://www.gnu.org/licenses/>.
*/

import org.bukkit.Location;
import org.bukkit.util.Vector;

public class Vector3D {
    /**
     * Represents the null (0, 0, 0) origin.
     */
    public static final Vector3D ORIGIN = new Vector3D(0, 0, 0);

    // Use protected members, like Bukkit
    public final double x;
    public final double y;
    public final double z;

    /**
     * Construct an immutable 3D vector.
     */
    public Vector3D(double x, double y, double z) {
        this.x = x;
        this.y = y;
        this.z = z;
    }
    
    /**
     * Construct an immutable floating point 3D vector from a location object.
     * @param location - the location to copy.
     */
    public Vector3D(Location location) {
        this(location.toVector());
    }
    
    /**
     * Construct an immutable floating point 3D vector from a mutable Bukkit vector.
     * @param vector - the mutable real Bukkit vector to copy.
     */
    public Vector3D(Vector vector) {
        if (vector == null)
            throw new IllegalArgumentException("Vector cannot be NULL.");
        this.x = vector.getX();
        this.y = vector.getY();
        this.z = vector.getZ();
    }
    
    /**
     * Convert this instance to an equivalent real 3D vector.
     * @return Real 3D vector.
     */
    public Vector toVector() {
        return new Vector(x, y, z);
    }
    
    /**
     * Adds the current vector and a given position vector, producing a result vector.
     * @param other - the other vector.
     * @return The new result vector.
     */
    public Vector3D add(Vector3D other) {
        if (other == null)
            throw new IllegalArgumentException("other cannot be NULL");
        return new Vector3D(x + other.x, y + other.y, z + other.z);
    }
    
    /**
     * Adds the current vector and a given vector together, producing a result vector.
     * @param other - the other vector.
     * @return The new result vector.
     */
    public Vector3D add(double x, double y, double z) {
        return new Vector3D(this.x + x, this.y + y, this.z + z);
    }
    
    /**
     * Substracts the current vector and a given vector, producing a result position.
     * @param other - the other position.
     * @return The new result position.
     */
    public Vector3D subtract(Vector3D other) {
        if (other == null)
            throw new IllegalArgumentException("other cannot be NULL");
        return new Vector3D(x - other.x, y - other.y, z - other.z);
    }
    
    /**
     * Substracts the current vector and a given vector together, producing a result vector.
     * @param other - the other vector.
     * @return The new result vector.
     */
    public Vector3D subtract(double x, double y, double z) {
        return new Vector3D(this.x - x, this.y - y, this.z - z);
    }
    
    /**
     * Multiply each dimension in the current vector by the given factor.
     * @param factor - multiplier.
     * @return The new result.
     */
    public Vector3D multiply(int factor) {
        return new Vector3D(x * factor, y * factor, z * factor);
    }
    
    /**
     * Multiply each dimension in the current vector by the given factor.
     * @param factor - multiplier.
     * @return The new result.
     */
    public Vector3D multiply(double factor) {
        return new Vector3D(x * factor, y * factor, z * factor);
    }
    
    /**
     * Divide each dimension in the current vector by the given divisor.
     * @param divisor - the divisor.
     * @return The new result.
     */
    public Vector3D divide(int divisor) {
        if (divisor == 0)
            throw new IllegalArgumentException("Cannot divide by null.");
        return new Vector3D(x / divisor, y / divisor, z / divisor);
    }
    
    /**
     * Divide each dimension in the current vector by the given divisor.
     * @param divisor - the divisor.
     * @return The new result.
     */
    public Vector3D divide(double divisor) {
        if (divisor == 0)
            throw new IllegalArgumentException("Cannot divide by null.");
        return new Vector3D(x / divisor, y / divisor, z / divisor);
    }
    
    /**
     * Retrieve the absolute value of this vector.
     * @return The new result.
     */
    public Vector3D abs() {
        return new Vector3D(Math.abs(x), Math.abs(y), Math.abs(z));
    }
    
    @Override
    public String toString() {
        return String.format("[x: %s, y: %s, z: %s]", x, y, z);
    }
}

Code in lines 54 and 89 is deprecated, is this possible now?
Line 54: "new PacketAdapter(this, ListenerPriority.NORMAL, PacketType.Play.Client.ARM_ANIMATION) {"
Line 89: "PacketContainer useEntity = protocolManager.createPacket(PacketType.Play.Client.USE_ENTITY, false);"

And I'm getting following errors when hitting an hidden player: http://pastebin.com/TH1XgrjC
This error points to "useEntity.getIntegers()." in my code, here it is line 90. What have I to do now to fix this error?

@comniemeer: Try this version instead:
https://gist.github.com/aadnk/d4f5d49fe079cb13da9c