Use Custom Deals to overcome the 100 variant (or product) limit in Shopify discounts. Specify which variant titles or SKUs you want to include (or exclude) from the sales promotion.
Use Case #1: Apply a discount on all sizes "medium" in-store (Watch the 1st video on the left).
Use Case #2: Exclude a specific variant but discount all the other variants (Watch the 2nd video on the left).