In your code, the part for checking the group is fine exept for the CollisionShape3D() at the beginning. (This should actually throw an error because CollisionShape3D does not have a constructor)
You should call is_in_group
on the object that the script is on, so just if is_in_group("Enemy"):
(or I suppose if self.is_in_group("Enemy"):
if you prefer that way) would work.
The first step doesn’t seem to be implemented though, and you still have the _on_mouse_entered(): ...
from before. Or is it just in another script and you didn’t delete that part yet? In which case I wouldn’t know why the script extends Area2D which is a 2D node that doesn’t belong in a 3D world.