You should be using mega evolution on your main team.
Mega evolution makes the most powerful pokemon even more powerful.
Gengar and Alakazam are the fastest special sweepers in the game and they also have mega forms which makes them ridiculously powerful.
4 Evs = +1 stat increase at level 100.
There is a limit of about 252 Evs for each stat and a max EVs of about 510.
So pick two stats, usually attack and speed or special attack and speed, and max them out through the super training games.
Yes natures and IVs are important for getting higher stats.
I have an Excadrill with an Adamant nature, top attack IVs and fully attack EV trained, and it gets its speed doubled in a sandstorm. It hits like a truck.