You could have the invisible wolf not as a separate object type but as a normal wolf object, and have an instance boolean variable that tracks if it is an 'invisible' variant. When it gets shot by the observerTower, just toggle the flag (to make it shootable by normal towers) and bump its opacity to 100. If I've understood your needs.
Another thing (more general) is to have more families than just 'enemies'. e.g. 'shootable' or whatever. You can have objects in more than one family to give you more control. It's 'kind' of like dealing with a class hierarchy that way.. kind of.