Quote from st0rmieHey, a question for you: having done the unique collection, what would be your wild-ass guess for chance of a unique spawning?
Its very hard to say, some uniques are harder to find than others but its very random, some have had problems with uniques ive found several times and ive had problems with some that are known as more common. The hardest ones for me were probably Otzi, Saha the Slasher (maybe due to it being bugged pre 1.0.5) and Red Rock. Saha was brutal and took many hours over days of game creating.
If you are aiming for all achieves I suggest hunting them as I did, make a list and when you are leveling 2 of each character for the class achievements and only level in the zones you need uniques in.
And just to clarify, 1.0.5 did NOT fix champion and rare achievements, some mobs were fixed but not even close to all. They even bugged some new ones in 1.0.5, fallen cur spawn in ancient path was removed without being added to another zone, so if you didnt get them pre 1.0.5 you are out of luck.