Curious Expedition 2

Party members can receive mental ailments (e.g. Paranoid, Cannibalism, Acrophobia, etc. etc.) from random events, most reliably Insanity events that occur when your sanity is at 0.

Unfortunately Insanity events also have a (quite likely) chance to kill party members, which can make getting 6 ailments difficult in normal circumstances.

However, the Inspector (in director mode) and Victoria (in certain campaign missions) cannot be removed or killed from the party except in extreme circumstances, which makes them perfect for this achievement.

To unlock this relatively easily, dismiss everyone else from your party until it is just you and either the Inspector or Victoria, and then keep triggering Insanity events until they have gained a total of 6 ailments.

This can be risky on higher difficulties, but when they are the only other party member the fail events are relatively rare meaning you should hopefully not have too much trouble unlocking this. You will likely however need to restart the run as you will be left with a terrible party to continue with.