Looks like there are finally some optional quests to do here. First, you should begin to expand your base a bit. I recommend two more ghouls chopping wood. Sylvanas will attack your base from the north so have Arthas ready to defend with the other units you start with. Before too long Sylvanas will send out the first runner to Silvermoon and you will be given control of your first gargoyles.


Optional Quest: Silvermoon Runners
The runners will be running from Sylvanas' base to Silvermoon. If a runner reaches the base the Elves will be alerted to your presence. Keep the group of gargoyles on the ready to deal with them as they come. There will be a countdown timer before each runner is sent so you'll know beforehand. Keep the gargoyles out of harm's way until a runner comes. If gargoyles get damaged a lot they can use the stone form ability to heal themselves pretty rapidly. Then, make them come out of stone form when the runners come. This will happen throughout the entire mission so you must constantly deal with it until you destroy Sylvanas' base. Also, the third runner I believe caught me by suprise by heading the other way ... it didn't make it very far but you should pay attention to where the runner is coming from.

Optional Quest: Destroy Sylvanas' Base
With the base we started with being pretty well developed at the start there are only so many things you must do before you can assault Sylvanas' base with a decent attack force. Upgrade your necropolis to a black citadel and get the ghoul frenzy ability for your ghouls. This improves their attack speed by 25%! Don't forget that you must research the web ability for your crypt fiends this time. Bring at least three of them, five ghouls and three necromancers with Arthas to assault the base of Sylvanas. Some gargoyles could join as air support and don't neglect the meat wagons, either. Upgrade these units until you feel comfortable.

Scout out the path you must take with the gargoyles that have been killing runners while you have a chance in between their coming. Then, begin the march. Head southwest across the bridge ignoring the Elven village you come across. Then, once you cross another bridge head north. Have the meat wagons focus their fire on the defensive structures as usual and keep Arthas healing units (as usual) this time once you've destroyed air defenses you can bring some gargoyles in to help deal the damage as well. Sylanas' base isn't very powerful fortunately and you can finish it off without too much trouble. You may want to have Arthas focus on Sylvanas herself with his death coil attack as she is the most dangerous thing you'll face here. When you've finished off the base a cutscene ensues and Arthas turns Sylvanas into a banshee - you can build them at the base now. Also, you can get the hood of cunning which increases intelligence and agility +4. Cool.

With Sylvanas' Base destroyed it's time to focus on the main mission. Replenish your forces at your base and march your forces south. You could use some gargoyles as scouts for this as they're pretty fast. You could take a couple banshees for support casting of their curse spell, which makes the enemy miss 25% of the time. Put the banshees in the group with the meat wagons if you like. At any rate, you can now proceed to assault the main base here and it's pretty well defended. This will probably be the hardest assault yet. Near the center there are the guardians. Make sure your meat wagons take out the four towers here because they'll be a real hindrance to your success. Each guardian has 1500 hit points and can attack land or air units. Not only that, but they hit hard and there are four of them. I wish you the best of luck.

Take everything you have in here. Just don't leave your base undefended as there is a hero for the Silvermoon base who can cause some huge damage to your base should you fail. The guardians have the heavy armor type thus they take more damage from piercing attacks. I made a group of ghouls to go in and distract them (they won't last long) along with Arthas. My second group consisted of crypt fiends, banshees and necromancers as they all have piercing type damage. I made them focus their fire on the guardians one at a time while the others defended them. The necromancers should be set to auto cast raise dead as well because your fallen can become reinforcements. It may take a couple tries but the guardians don't come back when they're defeated so just work them until you win.