Instructions
Turn on your PC and select "Withdraw."
Select the Potion that is in your PC and withdraw it into your items.
Select your Pokemon at Professor Oak's laboratory. Choose whichever one you like the most, because no matter what, your Rival will pick one that has an advantage over it, so you are unable to pick one that is better than your Rival's.
Battle your Rival after both of you have your first Pokemon. Continuously attack his Pokemon while making sure to keep your Pokemon's health up.
Use the Potion you withdrew from your PC if your Pokemon's health gets too low. This will instantly heal your Pokemon for 20 health, which will make defeating your Rival easier.