Requirements:

2xCross
1xcut2
4xCrush2/3

In th Fall City-Ring Town tunnel, around where the Zubat are on the left, is a crossing post, cross here with a cut2 and 4 crush2/3. Cut the metal fence, crush the boulders then go collect some of these along with a cross:

.Blaziken
.Typhlosion
.Raichu
.Electebuzz
.Haryama
.Machoke
.Machamp
.Pha npy
.Donphan
.Etc

Make sure Minun/Plusle are well charged, this could take a while. For going back, there is a Tangela outside the door to Registeel. Have fun!