SkillExecutionPacket:
===
package mcd.rivenmod.packet;

import io.netty.buffer.ByteBuf;
import mcd.rivenmod.main.RivenMod;
import mcd.rivenmod.skill.ISkill;
import mcd.rivenmod.util.RivenPlayer;
import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.entity.player.EntityPlayerMP;
import net.minecraft.util.ChatComponentText;
import net.minecraftforge.fml.common.network.simpleimpl.IMessage;
import net.minecraftforge.fml.common.network.simpleimpl.IMessageHandler;
import net.minecraftforge.fml.common.network.simpleimpl.MessageContext;

public class SkillExecutionPacket implements IMessage
{
    private int id;
    private EntityPlayer player;

    public SkillExecutionPacket() {
    }

    public SkillExecutionPacket(int id, EntityPlayer player) {
        this.id = id;
        this.player = player;
    }

    @Override
    public void toBytes(ByteBuf bb) {
        bb.writeInt(id);
    }

    @Override
    public void fromBytes(ByteBuf bb) {
        id = bb.readInt();
    }

    public static class SkillExecutionPacketHandler implements IMessageHandler<SkillExecutionPacket, IMessage>{
        @Override
        public IMessage onMessage(SkillExecutionPacket message, MessageContext ctx) {
            EntityPlayerMP serverPlayer = ctx.getServerHandler().playerEntity;
            int skillId = message.id;
            RivenPlayer rp = RivenPlayer.get(message.player);

            if(rp == null) return null;

            int cooldown = rp.getCooldownFromId(skillId);
            if(cooldown <= 0) rp.setCooldownFromId(skillId);
            cooldown = rp.getCooldownFromId(skillId);

            ISkill skill = RivenMod.skillList.get(skillId);

            if(cooldown > 0) return new SkillFailedPacket(false);
            else {
                serverPlayer.addChatMessage(new ChatComponentText("Skill Activated!"));
                skill.performAction();
                return new SkillSuccessPacket(skillId);
            }
        }
    }
}

---

RivenPlayer:
===
package mcd.rivenmod.util;

import java.util.ArrayList;

import mcd.rivenmod.main.RivenMod;
import mcd.rivenmod.skill.ISkill;
import net.minecraft.entity.Entity;
import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.nbt.NBTTagCompound;
import net.minecraft.world.World;
import net.minecraftforge.common.IExtendedEntityProperties;

public class RivenPlayer implements IExtendedEntityProperties
{
    public static final String EXT_PROP_NAME = "RivenPlayer";

    public static final int Z_COOLDOWN_WATCHER = 20;
    public static final int X_COOLDOWN_WATCHER = 21;
    public static final int C_COOLDOWN_WATCHER = 22;
    public static final int V_COOLDOWN_WATCHER = 23;

    private int zCooldown, xCooldown, cCooldown, vCooldown;

    private EntityPlayer player;
    private ArrayList<ISkill> skillListInstance;

    public RivenPlayer(EntityPlayer player) {
        this.player = player;
        skillListInstance = RivenMod.skillList;
        zCooldown = 0;
        xCooldown = 0;
        cCooldown = 0;
        vCooldown = 0;

        this.player.getDataWatcher().addObject(Z_COOLDOWN_WATCHER, zCooldown);
        this.player.getDataWatcher().addObject(X_COOLDOWN_WATCHER, xCooldown);
        this.player.getDataWatcher().addObject(C_COOLDOWN_WATCHER, cCooldown);
        this.player.getDataWatcher().addObject(V_COOLDOWN_WATCHER, vCooldown);
    }

    public static final void register(EntityPlayer player) {
        player.registerExtendedProperties(RivenPlayer.EXT_PROP_NAME, new RivenPlayer(player));
    }

    public static final RivenPlayer get(EntityPlayer player) {
        return (RivenPlayer) player.getExtendedProperties(EXT_PROP_NAME);
    }

    @Override
    public void saveNBTData(NBTTagCompound compound) {
        NBTTagCompound properties = new NBTTagCompound();

        properties.setInteger("Z_COOLDOWN", this.player.getDataWatcher().getWatchableObjectInt(Z_COOLDOWN_WATCHER));
        properties.setInteger("X_COOLDOWN", this.player.getDataWatcher().getWatchableObjectInt(X_COOLDOWN_WATCHER));
        properties.setInteger("C_COOLDOWN", this.player.getDataWatcher().getWatchableObjectInt(C_COOLDOWN_WATCHER));
        properties.setInteger("V_COOLDOWN", this.player.getDataWatcher().getWatchableObjectInt(V_COOLDOWN_WATCHER));

        compound.setTag(EXT_PROP_NAME, properties);
    }

    @Override
    public void loadNBTData(NBTTagCompound compound) {
        NBTTagCompound properties = (NBTTagCompound)compound.getTag(EXT_PROP_NAME);

        this.player.getDataWatcher().updateObject(Z_COOLDOWN_WATCHER, properties.getInteger("Z_COOLDOWN"));
        this.player.getDataWatcher().updateObject(X_COOLDOWN_WATCHER, properties.getInteger("X_COOLDOWN"));
        this.player.getDataWatcher().updateObject(C_COOLDOWN_WATCHER, properties.getInteger("C_COOLDOWN"));
        this.player.getDataWatcher().updateObject(V_COOLDOWN_WATCHER, properties.getInteger("V_COOLDOWN"));
    }

    @Override
    public void init(Entity entity, World world) {
    }

    public final int getCooldownFromId(int id) {
        switch(id) {
            default:
                return 0;
            case 0:
                return getZCooldown();
            case 1:
                return getXCooldown();
            case 2:
                return getCCooldown();
            case 3:
                return getVCooldown();
        }
    }
    public final void setCooldownFromId(int id) {
        switch(id) {
            case 0:
                setZCooldown();
            case 1:
                setXCooldown();
            case 2:
                getCCooldown();
            case 3:
                setVCooldown();
        }
    }

    public final int getZCooldown() {
        return this.player.getDataWatcher().getWatchableObjectInt(Z_COOLDOWN_WATCHER);
    }
    public final void decrementZCooldown() {
        int cooldown = this.player.getDataWatcher().getWatchableObjectInt(Z_COOLDOWN_WATCHER);
        if(cooldown > 0) cooldown--;
        else cooldown = 0;
        this.player.getDataWatcher().updateObject(Z_COOLDOWN_WATCHER, cooldown);
    }
    public final void setZCooldown() {
        this.player.getDataWatcher().updateObject(Z_COOLDOWN_WATCHER, skillListInstance.get(0).getCooldown());
    }

    public final int getXCooldown() {
        return this.player.getDataWatcher().getWatchableObjectInt(X_COOLDOWN_WATCHER);
    }
    public final void decrementXCooldown() {
        int cooldown = this.player.getDataWatcher().getWatchableObjectInt(X_COOLDOWN_WATCHER);
        if(cooldown > 0) cooldown--;
        else cooldown = 0;
        this.player.getDataWatcher().updateObject(X_COOLDOWN_WATCHER, cooldown);
    }
    public final void setXCooldown() {
        this.player.getDataWatcher().updateObject(X_COOLDOWN_WATCHER, skillListInstance.get(1).getCooldown());
    }

    public final int getCCooldown() {
        return this.player.getDataWatcher().getWatchableObjectInt(C_COOLDOWN_WATCHER);
    }
    public final void decrementCCooldown() {
        int cooldown = this.player.getDataWatcher().getWatchableObjectInt(C_COOLDOWN_WATCHER);
        if(cooldown > 0) cooldown--;
        else cooldown = 0;
        this.player.getDataWatcher().updateObject(C_COOLDOWN_WATCHER, cooldown);
    }
    public final void setCCooldown() {
        this.player.getDataWatcher().updateObject(C_COOLDOWN_WATCHER, skillListInstance.get(2).getCooldown());
    }

    public final int getVCooldown() {
        return this.player.getDataWatcher().getWatchableObjectInt(V_COOLDOWN_WATCHER);
    }
    public final void decrementVCooldown() {
        int cooldown = this.player.getDataWatcher().getWatchableObjectInt(V_COOLDOWN_WATCHER);
        if(cooldown > 0) cooldown--;
        else cooldown = 0;
        this.player.getDataWatcher().updateObject(V_COOLDOWN_WATCHER, cooldown);
    }
    public final void setVCooldown() {
        this.player.getDataWatcher().updateObject(V_COOLDOWN_WATCHER, skillListInstance.get(3).getCooldown());
    }
}