Well, I've done some testing, and I'm fairly convinced that Charizard can solo the Taka + Zel fight. BUT it requires more setup than I'm currently willing to invest: if I level Charmeleon to 43 it will get Flamethrower; then I could evolve it and common candy it back to 35 and use Flamethrower + substitute to deal with all of the Eeveelutions while Exeggcute hopelessly tries to leech seed me. Since the Grand Hall trainers are locked at about lvl 21 or so, I'm not too keen to do this! I tested the damage with Flame Burst and Substitute (hey, I like a good half hour of the Voltorb game every once in a while...!), and Glaceon's icy wind doesn't break the sub. I can always one-shot the Glaceon and inconsistently leave the Espeon with a sliver of HP. As it stands, I HAVE been able to knock out the Espeon before it could break the substitute by using smokescreen, but Flame Burst requires three hits to beat Umbreon, by which time the Exeggcute can leech seed and use two poison powders (I have a pecha berry to deal with the first), and the Umbreon uses confuse ray. If I had flamethrower, I could OHKO Espeon and 2HKO Umbreon, which would give me enough HP to win.
I guess another thing to try for Taka+Zel would be to reset the overworld weather until it's a sunny day -- does that weather condition extend to the Beryl Ward? -- that would definitely let Charizard OHKO the espeon and 2HKO the umbreon, and give it enough damage to solo the fight without Flamethrower. The weather in my game seems to be bugged at the moment, and anyhow I don't think I'm in the mood to get that cheesy just yet. A +SpAtk / -Atk nature with the IV's I've got would also probably give the required damage, without sunlight or flamethrower. So there are definitely several ways of doing this with a Charizard at lvl 35.
Anyway, I might mess around with some other starters, just to see if there are any other good possibilities. I wonder how far Feraligatr's evolutionary line + Meowstic could go in this game (I say its evolutionary line because we can't get Dragon Dance on the starter Totodile). Totodile learns ice fang at 20, and that with sheer force MIGHT be able to do reasonably well against Julia? ... but probably not.
I'll definitely leave this Charmeleon file backed up at multiple spots though. It could be fun to eventually do other things with the file, like see how few pokemon I would need to beat the game with a Charizard-started Fire monotype. (My guess would be Ninetales, Charizard (heat wave and solar beam), and Talonflame [you'd probably need to give Fletchling that lvl 45 Tailwind] could get you reasonably far, and then you'd have to decide what else to pick up.
And wow, that Torterra run sounds brutal, but really satisfying once you put in the work. Was the rest of the run so difficult once you got Squirtle? Oh and also, can't you get a Turtonator after Shade? (I'm not familiar with any of the versions other than E18, so I wouldn't actually know if this is a new feature.) ... What even is the full turtle team? Carracosta, Turtonator, Torkoal, Squirtle, Torterra; anything else?