does the same quantity discount apply to every item?
does every item in the store use the same quantity break point?
If so then you could set a calculation, using the abs() function to set the discount amount, then use the calculation in the discount tab.
For example, on the calculation tab, create a new calculation as:
TotalItemDiscount:
((abs([Quantity] >= 100 && [Quantity] < 500)?2.50:0) + (abs([Quantity] >= 500 && [Quantity] < 1000)?3.50:0) + (abs([Quantity] >= 1000 && [Quantity] < 2000)?4.50:0)) * [Quantity]
then define the discount on the discount tab as:
Trigger: Number of unique items in the cart
Calculation: Based on multiple of column subtotal
Subtotal of column "TotalItemDiscount" "times" "1" "plus" "0"