Things are already starting to happen when you take control. The base is small and can be quickly destroyed and one of your acolytes is already haunting the gold mine. Destroy the Elven town center first of all and construct your own there. Beware that the Elven hero is going to attack your base within a minute or two so you should leave Arthas and his party at home (there's not much for him to do yet). Build up your base and when ready build a few extra necromancers. They're useful with the raise dead ability. Whole hordes of skeletons can be raised, really turning the tide of a battle. Build ziggurats to the southeast, northeast and southwest entrances to your base. Upgrade them to spirit towers and you have your food supply/defensive structures all set up. Try to keep it so there's not any big holes the attackers could slip through as they will attack you occasionally.

You should make an attack force of at least three necromancers and seven ghouls, along with Arthas and a second group of about four meat wagons. Begin by attacking the base to the northeast. Don't forget to tell your necromancers to auto-cast raise dead! Use the meat wagons to clear the trees near the glowing blue insignia on the ground. When you're past this you'll get to an Elven hamlet. You can destroy this hamlet pretty easily with the attack force I recommended, and you're rewarded with access to the goblin merchant across the bridge to the southwest when you've destroyed it. I recommend you buy a scroll of protection and healing here if you have room in Arthas' inventory. You could haunt the gold mine here as well as it's a safe area to do so.

Southeast of the merchant lies a larger hamlet. This one did have a much larger defensive force and several of the magical defensive towers. Still, necromancers rock and they were overwhelmed by hordes of skeleton warriors. Continue on a path that tries to go southwest and you'll come across some light resistance. You can use leftover skeleton warriors from a previous battle to scout ahead. There is a place where you can destroy the trees to get to a fountain of health and a goblin merchant nearby. East of here I found a group of gnolls, one of which dropped a stone of health. Destroy their huts to find a book of the dead (raises 4 skeletal warriors and 4 skeletal archers).

Now continue to the west and you'll see that you're where the southwest path of your bas would lead, though you're behind the place where you'd knock down the trees to proceed, which is fine. Head southwest and you'll discover yet another elven hamlet. This one should be better defended than the previous two. Keep the necromancers healed with death coil, should they come under attack, and focus the meat wagons on any defensive structures you encounter. When this base is eliminated all that remains is the blue base to the east of here, and a pit stop to the south toward the fountain of mana. Destroy the golems here to get a potion of health that may be useful to you.

Replenish your forces back at your base if needed then proceed into the blue base. Don't go charging in, there are again towers firing at you here. Just focus the meat wagons on those and use the other units to protect them. This shouldn't be too easy but it won't be the most challenging raid either. Sylvanas is a little tougher on her own turf as she'll have the defensive forces to help her. I suggest focusing everything you've got on her as she is probably the most threatening thing you'll face. Use the book of the dead if you like and with the skeletal warriors your necromancers will raise they should be overcome by sheer numbers.