Not hard... just need some huge work. You need to give Herald a ring, to pass the ring guard you need to use special itens... you can find those itens in graves.
First thing to do is going in the church and listen to Herald, dont worry the undeads can't go into the church. After listen what he wants get out the church and get some waters from the fountain- like 30-35 just to make sure, you can allways go back and refill so dont worry. After get the water use it in the altar to make it holy.
With this water yu can kill the undeads, the skelletons dont need it. To kill them just give them the water while you hit them.
Now go to the building with a wood door. Kill the undeads and the skellie, take the key and go in library.
Here you will find the clues you need to find the clothes in the graves. Find the books (I think they are 5 of them), read the clues- writte them- and came back to the graveyard.
Try to find the graves, the itens you are looking for are IN the undeads bodys.
When you get all the itens use them and go behind the church. Enter the door and you will see a level to much (lol I think is a 102). If you are using the itens he will let you pass.
You are now in the non-magic zone. Here you need to find a ammy, with it you will be able to use magic.
Enter the door, you will see a undead walking around a central zone, wait until he turns around. That exactly spot is a secret wall... wait until he goes away and go in secret passage.
You will find several little rooms with one undead per room, just pass dont try to kill them... in the room with a red carpet, follow the carpet and click behind the wall, you will appear in the other side, in a room with several couffins. Open the one with blood on it. Take the ammy from the undead and use it. Now you are able to use magic! Go back to the room were you found the secret passage and follow the undead around the room. Go in the door at central wall.
Now you are almost finishing the lab... make sure you have enought holy water to kill the guys in couffins. Open one couffin at time. Don't open the central one!!!! You only can open the one at center after kill all the others undeads and skelletons.
Now that you kill them all open the last one, kill the guy, take the ring, and go back to the church. Give the ring to Herald and enter the gate. Grats you are now able to fight the gatekeeper.
TO KNOW: In non-magic zone you dont need to kill the undeads... they will turn away.