For this one, you need to complete Silke's Subquest in Beregost.
To start the quest, talk to Garrick in the town square, right next to The Burning Wizard Inn. He will lead you to Silke. To get the achievements, you must kill the "Thugs" when they show up.
Note that by doing this achievement, you will miss:
If you want both, save before talking to Garrick.