The best way to kill the fortune teller is to trap her the first time you meet her in the house in noer. Partners are again Gray and Cecily. With Gray on the right you stand on the left and clamp the fortuneteller in the middle.
Then proceed to hit and stun continuously from both sides to prevent her from casting spells. After killing the fortune teller, she enters stage 2 and is also her true form. At that point you run away back to the inn, where you team up with cocona.
Enter the house again, use summoning allies to keep cocona from getting too close to the fortuneteller's location and let her finish the job.
As for totems, when you equip them, they can summon a monster that attacks at close range or a ball of flesh that explodes and drains health slowly. Use those 2 to plan the fortune teller's blood. Because NPCs usually attack those 2 things will not attack you back. It works like a trap
P/s: Killing the fortune teller unlocks the achievement, but you also can't reopen the CG. So please consider