Create new discount rule, fill fields in „Discount info“ and save.
Select discount type „Assigned to manufacturers“.
In panel „Applied to manufacturers“ select manufacturer name, to which products discount must be applied.
In this case: "Metaxa".
Add in „Requirements“ panel new „Include / exclude products by sku“ requirement.
In section „Selected Categories“ select Category Action Type „Exclude“ and select category name, which products are excluded from discount.
In section „Selected Products“ select Product Action Type „Exclude“ and select products, which are excluded from discount.
Save requirement section changes.
Leave section „Selected Manufacturers“ empty.
Save discount rule.