func _on_Area_body_entered(body):
if is_in_group("Wall"):
queue_free()
Here, is_in_group("Wall")
will check if the node holding this script is inside the Wall group. It does not use the body
parameter at all.
But if you change it like this:
func _on_Area_body_entered(body):
if body.is_in_group("Wall"):
queue_free()
This will check if the colliding body is inside the Wall group. Choose the one you need.