This one is quite simple, just go to pray at the church once a day (It can be done any time of day) whilst checking the Sister repeatedly until she repeats. Her dialogue options will change after several days of prayer and switch to talking about the goddess letting you continue to live and such, at which point you should keep chatting with her until you get repeats again.
Keep praying daily at church and her dialogue options will change a third time. You'll know you're there when she talks about uh.. Showing cleavage while you pray. >> Exhaust all those dialogues (She has a lot of good insults for you) and the achievement should pop when she circles back around.