One way would be to turn shadows off on your models initially. When your player transitions from one floor to another, setup a trigger to change a floor variable as to which floor the player is moving to. The models can have an action to turn their shadows on if they are on the same floor as the player. This would still take some carefull planning to make sure the player can only change floors where there are triggers.


Never argue with an idiot. They drag you down to their level then beat you with experience