At any rate, if you're still resetting for first encounter with Yveltal (complete with cinematic "hatching"), you can faint him. After fainting, he'll still be there, and you can interact with him to start the battle instead of watching the cinematic again.
In battle, from the switch Pokemon page, you can select Yveltal and click "Summary" - that'll let you see his stats. If you use an IV calculator (just search "Pokemon Y IV Calculator", you should get Serebii), you can measure the preferred stats with preferred nature. Thus, no need to finish the battle.
Of course, do this after getting Synchro. It may take awhile to get Synchro, but it'll take much longer to wait for good IV Yveltal with randomly selected natures instead of 50% chance of nature you want.
Some people seek argument - sometimes it's best to deny them that by ignoring certain topics. In general, be kind.