Search the forum, this is an FAQ and answered in several different threads. The right setting is symmetrical in both directions, which doesn't leave you many choices (but heck there were only 64 to start with - a proper 'old school RPG would have at least 1024, or the switches would be in different caves, or whatever).
The skull is located in a chest in a building whose entrance is through the raptor area. You go in the main building through the gate, though the raptor area to the other building. You may have to kill a few raptors on the way.