Warcraft III: The Frozen Throne Walkthrough & Game Guide

Mission 2:  A Dark Covenant
The next day, at the alliance base camp, Prince kael'thas and his men ready
themselves to move out.

This mission is not as hard as it may seem.  You only have to destroy the green
undead base.  You will definitely want dragon hawks, lots and lots of dragon
hawks.   Also make some archers.

Let all of your bases not on the main island bases get mutilated.  You have no
choice, you can't defeat them.    Once they are all destroyed a cut scene with
lady Vashj will occur.

Now with the control of the naga forces you are given the option of getting an
expansion.

Optional Quest:
Acquire a gold mine
-secure a gold mine and build a town hall

This shouldn't be an optional quest.  Bring your naga and all the units you've
built, and a few workers to the base to the south.   Have your dragon hawks cloud
the towers and your naga and other units take care of the rest.  Then have your
workers build another town hall and put some workers on gold and tower up there as
well.
--
You can go creep around the main island for money if you so desire.  But defeating
the undead base isn't that hard, especially if you have been massing dragon hawks.
I had only 50 food worth of dragon hawks, workers, naga, and 4 archers, and this
mission was very easy.   Just put your units in ships and then unload then at the
entrance to the undead base at to the west of your base.    I suggest putting your
dragon hawks in a control group and your other units in a different one (or
multiple ones). Whenever you see a flying unit, use aerial shackles, and use cloud
on towers.   The naga should mop up the rest.   Make sure to focus fire on the
omni-annoying dreadlord (nuke him with flamestrike and forked lightning).  Cloud
the black citadel and then mop up the rest of the unit producing buildings first
and then destroy the other buildings and you will complete your mission.