Portions is probably not the best way to do it, certainly not the easiest if its a fixed discount for a product group.
If its a fixed discount I would set a update order action with a price change using a calculation/formula (would have to double check but sure its posible) ‘like price - 10’ on a rule which was for on order added with constraints for happy hour times and the product group of cocktails (if you have your groups setup like that).
Here is a good link for happy hour constraints.
Would have to hunt for the code/formula for the discount (the refund rule would be a good place to look for the correct tages as beleive that is 0 - ‘order price’ so could easily be coppied and adapted to order price - 10