I think nature is the easiest part to manipulate, using the everstone if i remember right on the parent poke with the right nature that you want. It will just be tedious around 30-45 minutes of running around catching pokes until you get one with the right nature
The thing is with pokemon with more perfect IV's the easier it is to cross and produce a baby with perfect stats. I think of it like the genetics punnet square we do in high school if you have ever taken a biology class
By the way can someone add me into their friend safari and tell me what I have in my safari please, the more safari friends the merrier :) !