Arthas is a little different now, no? His death coil attack is basically holy light in reverse as it heals undead and harms the living. This mission is more challenging than the first human mission and you can really slip up and get yourself killed. So be careful. Also, you can recognize the hidden acolytes by their reddish tint. First we're not going to do any sneaking around and head to the southwest toward the small town. There you'll find three acolytes. Kill everything here, including the villagers. When the battle is over you can heal your ghouls with their cannibalize ability to save Arthas' mana.

When you come to the stream, begin to cross it and you'll be granted a shade (invisible unit with a big sight range) to scout ahead. Take the shade into the next area and watch the guards patrol but don't attack them. You can get the acolytes here with no violence if you like, by simply taking Arthas alone while the guards are on the opposite side of the map (don't cross in front of the stationary ones) or you can be violent and try to single out the fools and earn yourself a ring of protection +2. This won't be easy but if you use the hotkey c to use the death coil attack on the enemy guards (or your undead minions when they are damaged) you can do this. It may be time consuming but the ring of protection might be worth it.

Wait until the patrolling guards are nearby then have Arthas and the ghouls ambush them. This will prompt them to all attack you. Focus your attack on them and heal your ghouls when they become weak (try to save them before they die) and you should be ok. Run away, if you have to, and regenerate some mana and health. It would probably be better if Arthas took some of the damage here, really. When you've defeated all three guards (pick up your health potion!) there are but two more standing in your way and one of them is unfortunately a knight. The knight doesn't have the ring though, the horse behind the barracks does. Unfortunately, to get to that horse you have to destroy the barracks so nothing short of a full assault will do.

But first, kill the villagers and claim the acolyte in the southeast corner. This should be enough experience points for Arthas to achieve his first level. And I would recommend the upgrade you choose be the unholy aura ability. It allows any friendly unit near Arthas to heal automatically. Further levels boost this effect. Also, it makes your units run faster which is very useful. Now you should attack the knight and footman. I recommend you start with the footman. He'll take less time to kill, and you can get him out of the picture. Try to have Arthas take the damage here since he can heal with his potion. If he's low on health, have him retreat while the ghouls stay and fight. This will switch the damage to the ghouls, who Arthas can then heal with death coil.

When the knight and footman are dead you can slowly destroy the barracks and then the horses behind it. You may have some trouble getting all three units to attack the barracks. Take the ghouls inside the fence since they are small and you can use all of your might. Claim your ring of protection +2 and be happy, that was a lot of work. Now, be sure to get the last acolyte here and we'll be on our way to the graveyard.

Optional Quest: Get Arthas to the Graveyard
This isn't so hard and we'll pick up several acolytes on the way. Scout ahead with your shade and make your way to the northwest of where the barracks stood and into the forest. This way you can avoid the gate guards. Scout with the shade and you'll see all the acolytes you have to find. If you see the villagers running off toward the guards just run away. Scout the area with your shade and return when it's safe. You may want to kill the knight here while he's at the northern end of his patrol area. It should be safe and not too difficult for you. Now you should have eight acolytes found and the graveyard is to the east. When you enter you get some extra units that should be pretty helpful to you.

Now to get the final 12 acolytes. This isn't too hard nor any different from the rest of this mission. Just take your shade and scout out the acolytes. Don't break down the gate yet. Get everything outside it first. There is a town center which can be bothersome as the units there will turn into militia. Fortunately you went to the graveyard and have your own little army. Just use the death coil ability to heal any units that get damaged and when you're done with the militia destroy the town center. Note the group of bandits to the northwest. They are holding an acolyte and the bandit lord has a spider ring which increases your agility +1 and there is also a healing potion around here.

Now we're ready to bash the gate down. You should have 12 acolytes when you do this. If you don't, you missed one somewhere. attack the gate with all your forces and prepare to be attacked. Your 13th acolyte comes running out being pursued by two guards. Kill them and make your way into the city. Now you can basically do some running around with Arthas and grab the rest of them. Scout out the path you want to take and run it. There are four acolytes you can nab at once all the way in the northeast of the area. While you wait for your 20th acolyte to register just keep running away.