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Don't worry about catching them now. Everytime you beat the Elite 4 any legendaries that you haven't caught respawn. Come back later once you have Sleep + False Swipe.
No, you cannot catch a lvl 50 before beating the Elite Four. Though I'd be more worried about beating the Elite Four if you can't kill Azelf/Mesprit/Uxie. Only thing to do is level up your mons.
I don't need a shiny. I don't even need to take the pokemon from you. Just get into the Wi-Fi room with any random pokemon with your OT and open a trade with me. I have an AR code that will show me your SID.