The only way to exclude a product from a discount is to set up product-specific discounts for each of the other products that DO qualify for the discount. If you are conducting this sale with a Discount Code that buyers must enter, you could use the same Code for each of those discounts, just take care that you give each one a unique Discount Name (as that's how our system distinguishes one discount from another).
Another approach you might consider is just temporarily marking up the price of the bundle product to compensate for the discount, so its "sale" price would be the same as its regular price before you started the sale. You can just edit this product's price setting in Seller Admin, and your existing buttons for that product should pick up the new price.
