Warcraft III: The Frozen Throne Walkthrough & Game Guide

Mission 5: Dreadlord's Fall
At that same moment, back in the Plaguelands, Sylvanas and her loyal minion plan
their next surgical strike against the dreadlord's power.

There are two ways to do this mission.  You can either  pick one of the bases to
start off with or you can try to hit both of them.  I am torn between the two
options but I slightly favor going to one base, possessing and charming the good
units there and killing detheroc or garithos and then going to the other base and
going the same thing
I usually start off going to the undead base and getting the frost wyrm and
abominations and then killing Detheroc and then speeding to the other base to get
the knights and the gryphons and then kill garithos.  This means no heroes coming
to your base.  Which means you probably don't even need to Town Portal to your
base if you have towered well enough.

Then just keep sending banshees to your people inside of the bases and you should
be fine.  Just keep some ghouls at your base just in case a meat wagon comes
along.    Just go and destroy both of the bases.  There should be very little
resistance.    Just keep sending units from your base, you wont be fighting
anything really that difficult and if it is you just need to possess more.

Once both bases are destroyed the mission is complete.