Normal Monotype: Kanto: Update #1
I started the game, sped through the starting events and caught a Pidgey.
Trained the newly caught Pidgey to level 10
Caught a Rattata and trained it to level 10
Defeated Blue for the 2nd time and caught a Spearow
Replaced Pidgey with Spearow
Continued to Viridian Forest, caught a level 9 Pidgeotto
Grinded a bit outside of Pewter City
Brock's Geodude was taken out by a few Fury Attacks by Spearow, although I had to use a Potion once.
For the Onix, I had Pidgeotto spam Sand Attack until Onix's accuracy couldn't go any lower.
Then I switched in Rattata and spammed Tail Whip until it's Defence couldn't go any lower.
Used Quick Attack 4 or 5 times and Onix was defeated.
Rattata Lvl. 12
Pidgeotto Lvl. 10
I don't plan on using Pidgeotto in my main team, and I'm probably going to ditch Rattata soon.