Once you reach the end of the Last Lands, you will have some dialog and such. Once all this finishes and a new path appears, you can either continue to finish the final boss, or you can take a portal on the east side to leave and come back after. At this point you are free to do either as the game continues on after the final boss, so you could do that and then go for this.
Once you have reached the final part of the game, this side quest will be available. Set your party to include Nidr and Setsuna and head back to Nive Village in the east. Head straight into the village and into the second house. Some story will happen and then you should leave the village and head to Dazzshire Woods (The woods are next to the village). Once you enter the woods, head west on the first intersection and a bit of story will happen. Follow this path, fighting the enemies and then see some more story before unlocking this trophy