Very Easy Level 100s
You need to be at the Viridian Gym point of the game, then you put two pokemon in the daycare just south of Goldenrod. Then go to the gym and find an arrow thet leads to the wall, so if you hold the button opposite to the arrow, you will go on a cycle of up, arrow, wall, up, arrow, wall; or something like that. Make it possibly endless. Place your DS on a nightstand or a safe and non-underfoot place, and plug it in to a charger. Now you can do two things, and they both work:
1) DPPt-used: Place a weight on the B button and the button you want to move in. Endless cycle! Woo!
2) HGSS new way: With touchscreen, touch the run-shoes button. Then place a weight on the button you want it on, the direction you want to run at the arrow with.
-Keep volume low so it doesn't disturb anyone.
-Tell parents so they don't turn off unsaved game.
-This will take a few days to a week depending on the level of the said Daycare pokemon.
-Put charger in DS or battery die and unsaved progress dies.
-Put the DS in safe place or it will be disturbed and cycle will die.
-On behalf of no EV training, the Pokemon's stats might be a few points lower.