Do you have to beat the elite four and iris to get the light stone and..

Yes you do.first you have to beat the elite four and iris, then after the credits and you have loaded the game again, you will arrive at your house.you are confronted by your rival who tells you that there is a pokémon on Victory Road that is beckoning you.you theN go to the champion leauge and go south.there will be an area near the end of victory road where a pokémon was blocking your path previously, it is now open go in there.follow the pokémon through the cave until you come to N's castle.(N is a pokémon)you then lead N through the castle until you come to the last room(throne room from last game)where you fight him.after defeating him in battle he gives you the light stone.then you go to dragon spiral tower(the very top)and catch Reshiram.after that N shows up telling you that reshiram chose your ideals, and that Kyurem is at the giant chasym.(juniper's father gives you the DNA splicers at base of the tower)then you catch kyurem.......hope this helps, but if you have already found this for yourself just forget it.