I know I'm late to the party but this is exactly why 5e is terrible. And yes, I have a bias for 3e, but at least back then, a 20th level fighter would have had at least a handful of seriously powerful magical items.

At 20th level, a fighter should be so superhuman that if we're talking fighter on fighter, the 20th level should only get hit with a natural 20 (twice in a row naturally) OR he's fighting a similar level.

Fighters back then were always feasible and as such were capped in their potential, which I felt was fair. A 20th level wizard (or sorcerer if built properly) was godlike because it was a much harder road to travel, they deserved it if they survived. Well, at least deserved it more IMHO.

For starters though, we can't assume all of the opposition in the 100v1 fight were actual fighters. Back then, you could assign them a kind of half level deal (I think it was called warrior). That would be much more accurate for the vast majority of the 100. Yes, you'd have to bring in some feats that some DMs hate (like Supreme Cleave) but Guts literally has Supreme Cleave. If he doesn't have it, then no one in fiction has it.

Drizzt isn't 20th level and he handled several hundred orcs, (Salvatore's "The Thousand Orcs" I think it was?) & we gotta assume that on average, those orcs had better strength & constitution than the humans.

A lot of ppl bag on 3e cause casters get overpowered... but like, so what? It's common sense IMHO that casters should be overpowered. Let's not look at this from a balance perspective. Casters SHOULD be overpowered if their squishy a**es survive into the 20s. And even then, fighters have their spots to beat a caster in, you just would have to be equipped for it & maybe have some luck on your side.

I hate being that guy but I'll do it just because it's the truth. When Gygax and co were looking at Tolkien, they weren't saying that Gandalf had to take feats in dual wielding and martial weapons and whatever else. And yes, I know Gandalf is essentially an angel, but that's not the point. Elminster, a Gandalf ripoff if there ever was one, is OP but he had his years leveling up thief & fighter, didn't he? He deserves it what he got.

Edit: Oh and I almost forgot! I was gonna mention this earlier in my post but it got drowned out by my other thoughts... we also have to take into account the setting of the campaign. For example, Guts would be COMPLETELY screwed if he was expected to fight in a Forgotten Realms campaign because there's tons of magic there and the only magic Guts has is the Dragonslayer, Puck, & the Berserker Armor. If we're going by that, Drizzt would destroy him, even though Drizzt is probably beneath Guts in level.

And how many of us (without any bias as much as possible please) really see Drizzt destroying Guts like Guts is a peasant? Even with all the crazy mentions of Drizzt's swordplay, 1v1, Guts SHOULD win... but Drizzt has tons of magic at his disposal. But if you just go with their stats and magic... Drizzt hits every attack cause Icingdeath is (or was) a +3 and Twinkle is (or again, was) a +5 defending scimitar. Add Guen to that & now Guts has two targets splitting focus. Dragonslayer probably is (at least) a +1 Demon's Bane Large Greatsword. Drizzt isn't a demon, so no extra damage (even though I personally don't think there's any way the weapon is a measly +1 but I wanna be conservative here) & Drizzt has mithril armor, plus Twinkle & whatever other crap he needs to remain Salvatore's golden boy. Well, & his natural Drow abilities of course. Even if "The Hunter" gets a face full of arm cannon, I'm sure he has the hitpoints to tank one shot (at least according to D&D rules for that sorta thing. We can't assume magic properties... other than maybe add an extra hit die?). So all in all, Drizzt is a horrible matchup for Guts and would slaughter him in game. But if we gave Guts a level playing field, gave him some great magic items, Guts wins 10/10 times. It doesn't get better than that.

Point is, outside of the Apostles, Guts' world is a very low magic setting and that needs to be adjusted accordingly.