Vermillion city. You will see that the stomped out flat-thing, has been replaced with a ridiculously tall building, with a small pedestal on top.
Fight the 4,926,782,194,001 Trainers with no items (other than the Toy). Make sure none of your pokemon faint, and have a good battery(you can't save). All the trainers (except for the last 1,000,000) have lvl 10 magikarps, and give you 2 Pk. After defeating the disgraces to humanity, place the toy on the pedestal. After hearing rumbling you will see.....