This doesn't answer your question (of how to make a subclass attribute visible to the parent class), but it should produce the same result - in a manner that easily expands. You can add more subclasses without going back and changing opponent
each time.
class opponent
constructor: (ID, Level, Name) ->
@ID = ID
@Level = Level
@Name = Name
@Health = if @Level is 1
@Level * 5
else if 2 <= @Level <= 4
(@Level * 6) - (@Level * 2)
@Luck = Math.ceil(@Level * 1.25)
@attackDamage = 0
# ...
class Snake extends opponent
constructor : (ID, Level, Name) ->
# use parent constructor to create the object
# and then customize the values for this class
super
@Luck = Math.ceil(@Level * 1.25) + 5