Where is a good place to train for the pokemon diamond E4

Question

Im through victory road and got up to the 3rd person of the E4 but i still need help where is a good spot to train.

Answer 1

walk around and fight abunch of trainers.

Answer 2

just train in the victory road or you can go to a secret route in victory which leads to the tetris piece shaped route in the map which has extra trainers and lv 5oish pokemon in the grass