I thought of Butterfree, but reliance on grinding it so it can spam Confusion and hoping Onyx doesn’t use Bind to get free damage is risky. If it uses Screech before Tackle it can really threaten your team. Geodude isn’t much of a threat though.
Decided to check Pikachu's level-up Moves in Yellow, and saw they added Tail Whip at Level 6, so I think they also realized the problem and tried to mitigate it best they could (of course, you'd had to have kept Tail Whip cause Quick Attack is Pikachu's 5th learned Move which means either Growl or Tail Whip is going bye-bye).
Of course, we're talking as if that if you lose you're game gets deleted and you've wasted days of progress. When infact this problem is just within the first hour of the game and, if you lose, you're just sent back to the last Pokemon Center you healed at. Infact it would be a recreation of the anime to lose the battle and then have to figure out a different strategy, though unlike Ash the player's options would be to explore the surrounding routes to see what options are available and discover the karate kicking Mankey and Nidoran.