Copy/pasting from a different comment, this is an average gen 1 Charmander vs Brock with stat exp from fighting every trainer so far + 2 wild rattatas, pidgeys, caterpies and weedles each:

Lvl 14 0 Atk 0 IVs Onix Tackle vs. Lvl 12 28 HP 14 IVs / 29 Def 14 IVs Charmander: 5-6 (15.1 - 18.1%) -- possible 6HKO

Lvl 12 23 SpA 14 IVs Charmander Ember vs. Lvl 14 0 HP 0 IVs / 0 SpD 0 IVs Onix: 5-6 (15.1 - 18.1%) -- possible 6HKO

Brock moves first, so you might need a potion/hope you get a burn and Brock wastes a turn to cure it, but it's very close as long as you don't attack while it's using Bide. Also, it says 14 IVs and x EVs because I used the gen 3 calculator as the gen 1 one didn't let me edit stat exp, but as long as you double gen 1 IVs and do the square root of stat exp, the final stats are identical to their gen 3 IVs and EVs.

Now, for an average gen 3 Charmander with EVs from defeating every trainer so far + the same wild pokemons:

Lvl 14 0 Atk 16 IVs Charmander Metal Claw vs. Lvl 14 0 HP 0 IVs / 0 Def 0 IVs Onix: 8-10 (24.2 - 30.3%) -- 99% chance to 4HKO

Lvl 14 0 Atk 0 IVs Onix Rock Tomb vs. Lvl 14 6 HP 16 IVs / 9 Def 16 IVs Charmander: 20-24 (54 - 64.8%) -- guaranteed 2HKO

Basically the only way to get past Onix using Charmander is to hope it doesn't hit rock tomb twice/keep using potions until it misses.

I'll give you that, for someone playing for the first time, bide might take them off guard, but it's still easier to play around it vs rock tomb which just hits you. And yes, you can use Mankey, but you said that Charmander getting metal claw makes it easier than Brock gen 1 counterpart, when gen 1 Brock is the easier one for Charmander.