In order to get this achievement you only need one of each hobbit (variation that is)
Bilbo - You can get 2 variations of him young Bilbo in Rivendell for 125,000 studs or old version in bag end for the same price
Frodo - there is 4 variations of him all are story related unlocks
Sam - There is 3 variations of Sam all story related unlocks along side Frodo
Merry - Has 2 variations both are story unlocks
Pippin - Has 2 variation both are story unlocks
Rosie - 1 variation and can be bought in hobbington for 25,000 studs
Once you have a variation of each of the above characters fast travel to Isengard walk through the gates once inside change character and do it over again with each character (You do not need tom bombidale for this).