>>14528875>>14528920My bud said he got one of every stone for completing all 6 mini-games of the Ev training on all 3 levels.
Also, they get much easier as you train the pokemon. Attack and SpA raise how many points you get per goal, Defense and SpD raise how fast your focus recharges (the little circle at the bottom of the upper screen), Speed affects how fast you move and how fast your soccer balls fly, and Hp lowers the amount of points you lose when getting hit.
Naturally, the easiest is probably speed. If you plan on training speed at all, I'd start there. Furthermore, start at level 1 and work your way up. I started a fresh pokemon on a level 3 game and damn near threw my Ds after 30 minutes.