Atelier Iris 3: Grand Phantasm

Use Nell's siren class and use the trick or treat on the monsters....

That's correct, but you need to have found the Siren Mana in Ishtar first as it is not found as part of the the story. It's at Sound Hill, a single room accessed from the western teleporter on 2nd level terrace north , the area with the eastern pond.

Once you have the Siren Blade then these monsters can be turned into monster cookies:

Bird Rider, Lizard, Lizard Lord, Lizard Shaman, Lizard Warrior - Dakascus,
Puni Rider - Valtessa/Dakascus,
Jin Rider - Posporia/Ishtar,
Caterpillar - Valtessa/Grimoire/Dakascus,
Dark Knight, Dragon Zombie, Living Armor, Mad Brownie - Grimoire,
Dragonewt - Distorted Dimension,
Fungo - Valtessa,
Golden Pig - Dakascus/Ishtar,
Kettle Spirit - Grimoire/Ishtar,
Lizard Wizard - Dakascus/D.D.

You can also find some one in a chest: in Dakascus, and Posporia/Kuma area.