Question: how to catch ditto in pokemon emerald?
Answer: Defeat the Elite 4 with some powerful Pokémon.
Go to the Fossil Maniac's house in Fallarbor town, near Meteor Falls. ... 
Go into the Desert Underpass. ... 
Catch Ditto and proceed to the end of the cave and get the other Fossil you didn't get from the Tower in the Desert near Lavaridge.