Book of Demons

This achievement is best to do in bossfights on casual difficulty level. You will need a lvl. 3 fear card (having magic perk of longer duration helps a lot) and defensive cards like shield, healing amulet or heal spell (all can be lvl. 2). Also make sure to unequip your claws and other damaging cards (you can make two loadouts, one pacifistic and one damaging). I advise having the Flaming leap card equipped for reaching more monsters or escaping tough situations.

If you fight against the Antipope, just focus on him and try to avoid all his minions. You can use the doors on sides of his area to get to a better positions to shoot him without harming his spawns. After a few phases he summons a horde of zombies and Slithering maguses. Don't interrupt them as they try to summon Corrupted priests. Once priests are there, they tend to cast spell teleporting all nearby monsters towards you. Use your fear card and let them supply you with zombies. You get the achievement after the fear effect ends.

If you did not unlock this achievement in the Antipope fight, you can try your luck in the final bossfight. About halfway through the Archdemon fight, he summons lots of monsters and casts the same teleportation spell like the priests. Just switch to your defensive loadout and activate your fear card.