Ok, this is my plan:
Since we can't see where the Shadow stays when isn't possessing someone, we can only wait the next victim.
After discover victim, we should carry she/he to the underground, to the place that receives the sunlight next to the Gazebo of Chaos, and imobilize the person bellow the sunlight, with a rope, or chain or a straitjacket. This way will have a halo of light around the possessed and below her/him, will stay the shadow. By one hand the shadow will stay stronger at the circle, the sun's light is responsable for make the shadow stronger, but at other hand it will be concetrated and in full presence.
In front of the victim will stay Phantasm into the darkness locked inside a cage with a magnifying glass on his hand, the cage will stay close to the light halo just enough to let his hands at the light when stretched.
Alone with the possessed and with the magnifying, Phantasm will concentre part of the sun light into a point and points it to the shadow, this point of light won't give strength to the shadow but only pain, and this way will weaken the shadow and let it very stressed.
I noted that the shadow stay tied to the victim until the person be able to release itself or be released by someone or something. Being the one who is harming the shadow and the only person around, makes Phantasm the only target to the anger of the shadow. With the arm in the light will be easier to the shadow reaches Phantasm and, if I'm not wrong, complete darkness can weaken the shadow so the darkness around the light halo could prevent the shade to escape for another place. Doing this Phantasm will untie the shadow from the victim.
Once the shadow reaches Phantasm, will have a moment before he be completely possessed, with this he hides inside the cage away of the light and warns about his success.
With the warning people will come to release the victim, they will also cover the cage with a dark and heavy piece of cloth and carry to the Gazebo of Chaos. In complete darkness Phantasm will be able to defeat his own shadow ... I hope.
Seph