Amasty Resolution Center

Buy N items in a category, Get N Free

I want to set up a promo for Buy 10 items of the same category, Get 2 Free from that category.  For example, buy any 10 nail polishes, get 2 more nail polishes free. Does not have to all be the same SKU nail polish, can be any color or brand.  

I believe Amasty can do this because I read this on the sales page as an example:
  • Buy 2 blouses for full price and get 2 more with a 15% discount;

I tried with percentage discount for product set.
I even tried Buy N Items for a discount (and applying a 16.66% to 12 items which is close enough), did not work.
In Mag 2 standard, I could set Buy 10, get 2 free but it only tabulated on identical SKUs. I am hoping Amasty can solve this.