Pokemon X and Y [3DS] - Page 61

you don't need to check nature in battle. All nature does is +10% to one stat and -10% to another. So suppose you want an adamant Yveltal then perfect stats would be 201, 166, 115, 136, 118, 119. Also synchronizers work on legendaries, so it might be a good idea to bring an abra or ralts with your desired nature to the fight.

edit: helpful hints for resetting. Use the capture pass power. Only with capture power 3 with quick ball is Yvetal guaranteed to be captured, so it might be worthwhile to spam the power on people beforehand. Have backup strategies in case you don't have the necessary capture power and the initial quickball fails, such as lower its hp and using other balls such as dusk balls. Also, a very helpful timesaver is to have a pokemon with a speed stat 1 lower than the desired speed you wish, and let it use a move on the legendary (for adamant/modest this is 118, for timid/jolly it is 130). If your pokemon moves first then reset right away. Similarly, having a pokemon with seismic toss (which deals guaranteed 50 damage) can quickly determine the HP and allow you to determine whether to reset sooner.