How to get your pokemon to their genetic max stats?

I am playing pokemon emerald right now, and I have spent quite a while training my pokemon. Some of them are level 100's right now. However, when I look at places like Serbii.net for their maximum stats, my pokemon do not match those stats. At first, I decided that I had to give my pokemon lots and lots of boosters like calcium, protein, etc., but the game would only let me give 10 of them to each pokemon, and only 1 hp up to each pokemon. What should I do?

Another question I have is, how to get pokemon to the "beneficial nature"?

There is a complex and arduous journey ahead of you if you are aiming for max stats. I'll begin with the beneficial nature bit. Your Pokemon's Nature has be one that raises the particular stat you want to raise. Here is a list of what the natures do: http://www.serebii.net/games/natures.shtml Next, is the IV factor. It is a number that is assigned to a stat a pokemon has. There is a formula to find out what this number is, but the higher this number is, the better it is. It will certainly get you a head start. The formula should be here: http://www.serebii.net/games/ivs.shtml (If you're no good at math, this page also has a link to a calculator that will do it for you) Finally, the thing that should really be your problem, EVs. Vitamins will give you EVs, yes, but not enough to max out a stat. To max this value, you have to fight specific pokemon that raise the EVs of that stat. There are a number of online pokedexes that tell how many EVs a pokemon will give and to what stat. Be warned: EVs can only go so far. That is why I suggested having the boost of favorable IVs first.