Most of the encounters in this are fairly easy and can be done in a single playthrough, though it's also necessary to get to the main shop. To do this, complete the Dark Loyalties quest as described in the Freedom Fighter achievement guide. After that quest is complete, you will see a new rope line down across the ravine to reach a series of caves. Once you've kill all the enemies inside and spoken with the merchant, the encounter will count as complete.
There is also a hidden outfit in Dokkerland by going over to the mountain with the skulls and following it all the way up and doing a leap of faith down to a chest. This is not required for this achievement, but does make other achievements easier.