I had success after a few tries with a Metapod, Ratatta and Pikachu. I lost the first 2 battles but they let my Metapod evolve. For the 3rd battle, I used Stun Spore (yellow button) straight away, then alternated with Potion (blue button) and Tackle (red button) every second move to knock out Geodude. I did the same with Onix, then my Pikachu took down Onix with Tackle after my first 2 mons fainted.