I tried to look up how Hidden Power is determined, but I'm still a bit confused. After beating all of round 1 on PS2 and all of the cups on r2 except for the Rival Cup, I've decided that rental pokemon won't cut it for the r2 gym leader castle. In one of the FAQs though, it says to use Zapdos with Hidden Power (Grass), but unfortunately I have no idea how to get a Zapdos that will have the grass version of hidden power. I have pokemon Yellow, Gold, and Crystal at my disposal. Please help.
If anyone has any suggestions of other pokemon to use in place of Zapdos or Mewtwo you're welcome to let me know. It'll be a lot of work to train all of my pokemon up to lvl 70, because I believe that's what Mewtwo is when you catch it and all of the gym leaders pokemon are the lvl of your highest lvl pokemon.