Well there is the fact that we don't know Balor ability as a Floor Boss. Sometimes a boss abilities can change how a fight goes like how the Black Goliath fight they had the firepower to massively injure and maybe even kill it but it survived due to it's regeneration ability.

I know this isn't a fight we know much about but there was Zeus Familia vs Behemoth that was slightly mentioned in AR. In the event it had Zard be the one to defeat Behemoth despite there being people stronger than him in Zeus Familia. From what Zard said about the event it was implied that he didn't have a choice to eat Behemoth since he said he did it for his family and he did what he had to do. So despite there being stronger people, ability wise Zard's Deus Ambrosia was the best shot they had at winning against Behemoth's poison. Similar thing happened in the non canon Grand Day event where Ais ability was the best shot against Behemoth's poison even though there were stronger people there. Well this is what I think so this can be completely wrong as well.

Of course the stats of a Floor Boss is also very important in order to have a chance to beat one but you can't exclude their abilities as well since pure stats isn't everything about Floor Bosses. It's even more important if the difference between the two sides are not that great. If we were to say that Balor was the same Level as Ottar then stat wise they should not be too different and their abilities would have a greater impact on the fight. Kind of like how two adventurer's of the same Level clash their stats and abilities make the difference between victory and defeat. With Ais vs Udaeus Ariel should have put Ais on a similar level of power as Udaeus but his sword and minions managed to push her into a corner despite being at a similar level of power.

Ottar vs Zard was more of a 1 Level difference fight and even then there was a lot of plot to help Ottar win that fight. There was no point in time where Zard couldn't have killed Ottar whether Round 1 or Round 2. Even in Round 2 he began a long speech and had time go on despite that making him weaker when he could've killed Ottar then. Someone who was weakened and got weaker as the fight went on, even held back at some points and Ottar having the power of plot and still barely managing to win. Ottar in AR acknowledged that and even though Zard tried to assure him that he beat Zard at his best, Ottar in EF later called his rank up fights to 6 and 7 as "unfair fights" meaning he didn't take what Zard said to him seriously

While Ais vs Behemoth in Grand Day event is a good example of what you say, Zald killing the Behemoth is not. It was said that he killed it in one strike and the skill increases his stats so the reason he was able to kill was his new gained power(the poison had nothing to do with it) and not being a better match ability wise.

That Zald "did what he had to do" could refer to him simply being the one that got the opportunity to deliver the finishing blow and/or that he did it because the Behemoth was about to kill someone and he prevented it by doing that.

That's what I just assumed about their fight from the details they got so I'm sorry bout that. Just thought why Zard specifically had to do it compared to everyone else in Zeus/Hera where there people who had better stats and had more power than he did

EDIT: I edited that part so that people know it's just my thoughts on it.

My guess is that since Ottar was a level 6 during the dark ages of Orario I would think that the guild temporarily stopped expeditions or prevented members of strong familias going deep into the dungeon as they're needed to fight Evilus on the surface.

Actually, Oomori said that the Guild still encouraged adventurers exploring the dungeon as magic stones were the main source of income. Finn, Riveria or Gareth(can't remember who specifically) even complained about it in Astraea Record(I think they also did in SO9).

Though, I don't see what does that have to do with the topic.

It was Allen who was complaning on how much of a pain in the ass it was to do Deep Floor expeditons and hunting Evilus at the same time and said they could get Evilus done a lot faster if the Guild didn't force expeditions

Though, I don't see what does that have to do with the topic.

I'm just making a theory that Ottar didn't fight the Balor as a level 6 because he leveled up when the dark ages of Orario started. Going to deep floors takes hours or more than a day to reach depending on how deep they go and how many times they stop for breaks. If Evilus decides to attack when Freya, Loki or Ganasha familia are in those deep floors it would take them a long time to get back to the surface to respond. Also it would be a very bad time for Ottar to waste his energy or risking his life soloing a floor boss.

We know that the Balor is for level 7 Ottar to fight, so level 6 Ottar wasn't a question, I think.

My bad. I assumed we were trying to theorize the timeframe Ottar fought the Balor as a level 6 or 7.

Oh, that makes sense.