I was lv 80 with the friend bow and platinum rank and it still took me 12 tries to get him. (he has a -10% join rate so you have to be well prepared to get him
Once you get the Friend bow and reach the save point with a Pokemon strong enough to take on Rayquaza. Save and just keep soft-resetting or turning on and off until you befriend him. That's how I did it... But I forgot what Pokemon I had.