Taxes Item Groups and Discounts

I have a group of items and would like to apply a discount to this group of items.  Here is the issue.  I apply the discount and the discount takes into account the entire group of items.  That's ok in the case of states where all the items in the group are taxable.  I then can apply tax to the discount and taxes are handled correctly.  How do I manage this group when all the items in the group are not taxable?  How can I make sure taxes are handled correctly for the discount when not all the items that