No you don't. I caught it after I beat the Elite Four. You can catch it before or after. It doesn't matter.
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The rare Pokemon which you can get before the Elite Four in Pokemon Platinum are Giratina, Mesprit, Uxie, and Azelf.
After you beat the elite four and the galactic biulding
yes you must beat the elite 4
He is waiting in the battle area to the right of the Pokemon center.
If you notice in the old episodes he never did the orange island he battled 1. the new ones hes only battled 1 so far