Act 5 Nilathak Boss maps are not so random. There are 3 ways to find him but you must found the way point.
so from way point "Halls of Pain":
And after you found the "To The Halls of Vaught" crypt and go in there are 4 ways to find Nilathak. The ways have some specific marks which tell you where is Nila.
Follow the images:
That's it :). Just Be careful there!!!