First off you need 5 players total. 4 for the investigators and 1 creature.
This achievement requires each human player being a Classic character.
Which unlocks at Rank 10 and so on for more characters and Rank 11 for the one and only classic creature.
ALL 5 NEED TO BE CLASSIC CHARACTER! EVEN THE CREATURE!
Also doesnt matter which classic character you pick, just make sure it has (Classic) next to the name of the character youre trying to use.