idk why everyone fusses on how hard brock is to beat on yellow and how u have to get nidoran, mankey or butterfree to beat him. I beat him with my pikachu alone, no cheats or nothin. Here is how I did it:

1. raise pikachu to lv17 (which didnt take long)
2. get a couple potions
3. When you battle him, first use double team 6 times in a row
4. now that he rarely hits you, use quick attack for all his pokemon
5. You may need to use a few potions, but other than that, your set!

Thank you for this totally useless information.