>>33410360>>33410368People always say it's Charmander, but it's actually Bulbasaur that has the hardest time with Brock. Unlike the other starters, it's stuck with only Tackle until 13. You have to overlevel Bulbasaur for it to take down Brock easily.
Meanwhile, Onix has such a low special that Ember does pretty high damage to it despite the 4x resist.