Unlocking Benco City is not hard. Just beat the Elite Four 7 times! I have'nt done this but my friend did. I even saw it.

Note: You will be able to capture Azurill, Marril, Pichu, Pikachu, Dratini, Aron, Lairon, Acsson, (a Azumarill, Raichu, and Aggron if you find them, for it is hard).