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Mark Stephens is right, a long rope is the key. But you can do it with just one person. Tie something to the end of the rope ( a trash can lid would work) and toss it on the roof higher than the disc you want to retrieve. Now move where you are standing so that when you pull the rope down it intercepts the disc and drags it down.
Maybe tossing the trash can lid will strengthen your arm. You could carry it with you always and become famous for your unique training style. Players will point at you and say to their buddies, "See that guy. He can beat you with a trash can lid!"
Have you heard of the sport called dwarf tossing?