Selfdiscoveries, not 100% confirmed, regarding game mechanics:
I'll start with maps, even though this is a thread about the monsters, since it'll give us some insight on creature-hunting. I'll use the cave/event "Crumbling Vaults" in 'Stinging Winds' ACT II as the example.
A lot of times, the maps are fixed. This is the case with 'Stinging Winds' which is split into two areas, the Khasim Outpost in between the two. In 'Stinging Winds' there are, as far as I can remember, three possible cave spawns: "Cave" (can't remember if this is the exact name), Chamber of Idols and Crumbling Vault. Most people will be looking for the the Crumbling Vault, since this cave is the rarest. Usually, while looking for rare caves, players will discover the map and then leave if they do not find the desired cave. It is, however, possible for a player to discover only parts of the map and then be sure that their cave isn't in the map. Depending on the area, only a set amount of caves can spawn at a time. In 'Stinging Winds' the player can leave after finding a "Chamber of Idols", since the Crumbling Vault will not spawn in the same game.
Long story made short, the same thing happens with some creatures. For example "Red Rock" (Savage Beast) in the Southern Highlands. Since the area is small, only one pack of champion/elite/rare mobs spawn at a time, thus if a player finds a group of champion Savage Beasts (or in this specific example, Moonclan Khazra), he can remake the game.
I'm note sure with how many maps this works, but it's certainly worth sharing. This is what I've discovered so far, farming for achievements. Obviously, the smaller the area, the quicker the details will be revealed.
Edited by Tolle#2599 on 6/19/2012 12:29 AM PDT