In your example there'd be no point in tfs having stats. If it would be balanced for those two characters to be able to do the same fight, but the weaker one only having slightly more of a challenge, then the stats would have to be small to the point of being flavor text, and would be restricted from stuff like permanently flying. You'd get like dragon arms having a bonus of 1 hp, otherwise fights wouldn't be slightly tougher for the weaker, it'd be sgt rufus thundercock, second of his name, having armor pen to compensate the player having possible tf armor bonuses so a basic pc tank would be ripped a new one. however it's a party based combat system, so if each party member has bonuses, then the pc could be a supportive class and do fine, but your tank and dps would be the ones doing the heavy lifting. You couldn't tank the same hits or hit the same enemies because your character wouldn't be able to compete with accuracy bonuses, dodge bonuses, armor pen. In theory you could, it'd seem like you did it as a weaker class but in reality you'd be better replaced with any companion, or you'd say "i did it! i used 15 elixirs and a ability to not die for a few rounds but i mean i did it!" >see 7 hour long ffx level 1 bosses. Similarly to how you can't build a tank with only physical abilities, have him put all his stats into charisma willpower and cunning, give him underwear, and have the game balance so he'd do just as well as ice knight brint. You could compromise and make him a paladin and give him a few "acceptable" tfs but you can't just have the balance scale to whatever a player would do as foolproof protection. Something has to be a loss condition, or tfs just don't have any stats, the power is transferred to other places, and you avoid all of that, and people can rp whatever they want however they want and not rely solely on their party or exploiting game mechanics.
I'm not saying combat should be exactly the same for the regular human and the 4' goblin, except that the latter has to use an extra potion. Assuming there are three difficulties of encounter: easy, medium, and hard; and that for a regular human, these are respectively: a breeze (you can go through several encounters like this without healing), a bit of a challenge (you'll need to heal every so often, or play more defensively to conserve HP), and a real challenge (you'll need to use consumables, ultimate abilities, etc., to get through it)—assuming this state of balance for a regular human, then for a weak goblin, the same three encounters might look like this: the "easy" and "medium" encounters would correspond roughly to the "medium" and "hard" encounters from the perspective of a human, whereas the "hard" encounter would not be possible, and your character would have to run away. Now, this is from the perspective of
traditional combat, i.e. swinging swords and flinging spells until your opponent dies, but seeing that this is supposed to be an RPG, there could be additional methods of resolving encounters. Maybe as a weak goblin, you can trick the "hard" encounters—which you're no match for in traditional combat—with a sleeping potion, or a rock trap, etc.
The only real problem is when it comes to bosses, since bosses are usually gating content, which means the PC
has to be able to get past them, but here too, the solution is just to include more non-combat solutions. In TiTS, you can sweet-talk your way through a lot of boss encounters to avoid fighting; or, you can just lose (and get a defeat sex scene), then move on: it's not like CoC2, where you're
forced into beating the boss, in traditional combat, otherwise you get a game over and have to reload.
The guy beat kas multiple times but it was meant to be a encounter you lost. The dev's responses were poor, they should have been a bit more appreciative someone put that much time and effort into their game, but it boiled down to being invulnerable, buffing yourself, and praying for a crit. Unless you built a very specific way planning for it, had revive items and the revive belt, it wasn't something doable. He also did it with a lust build I didn't see but had enough power to nuke that encounter. She had reasonable stats so you might feel like you did something but if you ever actually beat her there, there was 4th wall breaking text about you cheating. Him beating her wasn't the real problem, though. The problem was that exact formula could be used on any enemy in the entire game before the changes. Who cares about a superboss, a monster in the abyss, whatever, invulnerability, crit roll, if you fail to crit, revive and try up to 2 more times, job done. It was a hole in the armor of every future encounter basically unless they started giving everything ludicrous amounts of hp which would make people playing normally feel like everything was a sponge.
That's what I'm saying: they shouldn't balance the game around exploits. You think if they left that it, then everyone was going to start stacking invulnerability items, and saving and reloading every fight while praying for a crit? Of course not. It's a role-playing game, and anybody that's ignoring that for the sake of min-maxing and intentionally trying to break the game like this is going to be in the minority. And again, what's the issue if it's possible? It's a single-player game, so anyone breaking the combat system is ultimately just ruining their own experience—no one else is affected. Now, of course you don't want exploits to be lying out in the open, that just any random player could stumble upon them; but if it requires a specific build, as well as continuously saving and reloading for that one perfect fight, then it's highly unlikely that casual players will manage to ruin the game for themselves.
Also, again, there are difficulty options, which I admit are generally a crutch for poor balancing (basically the devs saying "we can't be bothered to balance our game, so here, do it yourself"), but in this case, if someone wants to make themselves an OP 10' monstrosity but still wants a challenge, they can turn up the difficulty, while someone who wants to play as a 4' goblin twink but still doesn't want combat to be a hassle can turn it down.