After defeating the Hydra in the Encampment, go back to Cassardis. Madeleine will be waiting at the Inn. Talk to to her to start the 'Guard Duty' quest. You'll have to escort her to the Encampment. Go in the morning to avoid most monsters. Make sure she doesn't die. When you're back at the Encampment, Madeleine will ask for 1.000 Gold. Give it to her.
(EDIT: If you didn't escort her to the encampent, but you did complete 'Chasing Shadows' she will still open up her shop in Gran Soren. Go talk to her, the trophy might just unlock! This has happened for quite some people.)
When you arrive in Gran Soren, complete the 'A Matter of Myrmidons' Main quest. Afterwards, find Mason (black guy, pink clothing) at the Fountain Square. (If he's not there, rest a few days at the Inn or complete a few other quests first.) Talk to him to start the 'Chasing Shadows' quest. Go to the Inn and sleep till nightfall. Go to the Nobleman Quarters and a cutscene will start. Follow the knight, but don't get too close, or too far. Once you're done, go back to Fountain Square and talk to Arsmith to complete the quest.
Now rest at the Inn for a few days, until the 'abandoned house' marker disappears from where Madeleine her shop will be. (It's where you saw her talking to the knight.) If it doesn't disappear within a few days of sleeping, go do a few quests and come back later. Once her shop is there, speak to her and choose to buy. You don't have to actually buy something, but this will make the trophy/achievement unlock.