This solution is ok, but only works under very limited conditions (not too many items, variant being the only condition used).
It would be wonderful to have a discount for variant (or any condition) a product meets. Basically, any field used in the product entry should be available as a condition for discount (weight, physical or not, name, variant, etc).
If that cannot be done in a near future, would it be possible to change the limit or remove it completely? It was a basic function in Magento and I was suprised to see how limited the discount tool is in Shopify, discounts being such a big deal in e-commerce marketing.
I keep my fingers crossed that sholify will improve it so I don't have to try various apps that risk "breaking" things.