When fighting them as a collective boss battle, all 3 enemies have an immunity to Ice and Wind attacks, leaving Yukari, Mitsuru, and Metis unwise choices for the battle. The Seekers are resistant to physical attacks, giving Junpei difficulty in doing decent damage while simultaneously leaving him vulnerable to their (Ma)Garudyne attacks. This leaves Ken, Koromaru, and Akihiko as the few party members who can strike the Seeker's weaknesses with a passable Magic stat.
As the Seekers have Hamaon and Mudoon with accompanying Boost skills, ensure the party has a good stock of Homonculus or is equipped with items that defend against these skills. Maintaining Masukukaja and Masukunda will help in countering the Seekers' Evade skills that guard their weaknesses.
Acheron Seeker appears on Path 7 of Yomotsu Hirasaka within the Midnight Channel. After being defeated, it may drop a Mysterious Eye. Selling 10 Mysterious Eyes at the TV extension of Daidara will unlock the Shitisei Jakotsu for Teddie.