When you first enter Feldepost's Inn in Beregost, one of the patrons, Marl, will start to provoke you. For this achivement, you need to calm him down by talking to him.
You need to choose the following conversation options: 1 - 1 - 3 - 3 - 1.
Note that by doing this achievement, you will miss:
If you want both, simply save before entering Feldepost's.