I am not entirely sure about that but I will share some experience.
I go to the numpad 1 (SouthWest) direction everytime, if he is there then kill him.
If I see a champion then I will move tonumpad 9 (NorthEast) direction.
Now I am not sure about the following:
If I see a evil urn, I will teleport to numpad 7 (NorthWest) direction.
If I see nothing there, I will teleport to numpad 3 (SouthEast) direction.
I will comfirm this today.
You have to identify a blue wall in a certain location, that is where Nihla will be.
The best way to find out which walls I guess is after killing him checking the entrance to the "arm" where he was. Basically it is the first tile of wall in the middle piece that starts his corridor ("arm"). I hope this clarified a little
IOW, clockwise with random starting point, it's: no boss, champions, unique from urn, super unique (Nihlathak). From this you determine which direction to go once you check the lower left section, and you check that section because it's closest and monsters there will greet you upon entering, so it's easier dealing with them immediately. So, two wings of the Halls of Vaught is all you should ever need to see, one if the 25% chance he's in the lower left succeeds.