I'd say that both of them were at least level 10 so that they could pose a real challenge to the Zeus and Hera families who had level 9s (taking into account pseudo levels).

The precedents of Udeaus being defeated by Ais (pseudo level 6) and Goliath being defeated by Lefiya indicate/expose that a giant monster like a floor boss can still be defeated by a single adventurer of its same level. Therefore, for a single monster to pose a huge challenge to a large group of adventurers it must at least out-level the most powerful of the group, as in the case of black Goliath against Ryuu and company.

@Longhatruong, the problem I see with your logic is that you're assuming that the Zeus family confronted Behemoth and the Hera family to Leviathan each on their own. But I don't see the point in them doing it that way since these families were famous for working as one body, just like Royman claimed Finn for not doing the same thing them did with the Freya family. Also, why would they divide their forces to face these missions?

Maybe if both missions had happened at the same time in different places, but I don't think that's the case because Ottar witnessed both fights (presumably); we know for sure that he was in the battle against the Behemoth because he saw Zard defeat it and we also know that he saw Leon splitting the sea with his attack, and what better scenario to split the sea than fighting against Levithan? So, if he witnessed both fights, they were not simultaneous.

Ottar didn't see Leon "split the ocean". You are getting mixing up that Hildis Vini's shockwave cut through the water of lake Caldera and that he commented it wasn't as strong as Leon.

And yes, of course they didn't split their forces. Both monsters are mentioned as having been defeated by both factions.

@Leinad91

That's why I only stopped at "at least level 9". I don't know whether Zeus and Hera working together or not. About Hera's level 9 captain, I already stated that Leviathan being level 9 could still worth as a level 10 threat in a sea battle, so yes, Hera familia still would have a lot of trouble fighting it. Pls don't bring Hestia Familia case to counter me, we all know they survived due to plot.

Ottar witnessing Leon using his slash is not an indication that it happened during Leviathan fight. Ottar in vol 18 also didn't compare his ability to "split the sea" with Leon. Ofc, unless I'm missing info somewhere, and you are talking about a different scene.

@Rigel31415

And yes, of course they didn't split their forces. Both monsters are mentioned as having been defeated by both factions.

So it's confirmed that Hera Familia participated in fighting Behemoth, and Zeus Familia participated in fighting Leviathan?

Material regarding Grand Day mentions both Zeus and Hera as the slayers of the Behemoth. I can't remember if it is explicity said in the LN too, but mention those because they are the ones I recall.

In SO6 the Leviathan is described as having been killed by both Zeus and Hera as well.

@Rigel31415, Ottar saw Leon to use his "attack" at some point, only then could he make comparisons between the two, but the question is where? In the coliseum? I don't think so because such waves that endanger level 6s like Mia would kill the spectators.

That led me to think that it would be more possible that Ottar had seen it against Leviathan where Leon could use it at full power without causing collateral damage; also since Ottar mentioned that his attack was not as good as Leon's right after seeing the effect it had on the lake, I inferred that the water was a comparison factor.

It's only circumstantial evidence so I should have mentioned it in the previous comment, however I see it as the most likely scenario, although of course, it might have been otherwise.

@Longhatruong, you're relating Leviathan to Amphisbaena, the latter is an amphibian, that's why it has one level on land and another different one in the water, however, Leviathan is a purely maritime monster (if its representation is faithful to its mythological version), for that reason I didn't see any sense in considering that it would have different levels depending on the environment because it's a monster that will always be in the water.

Even so, you could have a point, but in the opposite sense to what you're suggesting, let me explain, it's not that Leviathan is stronger in the water (it's always in the water) but that the adventurers are weaker in underwater fights, although thinking that they would lose an entire level for being under water I think would be too much. But the postulate is valid in any case.

At this point we enter the realm of speculation but I think the fight against the monster was from "land", I seem to remember that the school district, before being a school, was a warship created specifically to confront Leviathan on the high seas, so each combatant would be in their element, the monster in the water and the adventurers on "land", so each would have his real power during the fight.

@Leinad91

Amphisbaena, the latter is an amphibian, that's why it has one level on land and another different one in the water

Are u sure about this? I don't remember Amphisbaena being an amphibian or that it can move on to land. In the anime, it doesn't even have any legs iirc. And I dont think Amphisbaena has level 6 in the water and level 5 on land. It is always a level 5 but fighting it in the water is as hard as fighting a level 6.

it's not that Leviathan is stronger in the water (it's always in the water) but that the adventurers are weaker in underwater fights

This is exactly what I means. Amphisbaena( and likely Leviathan) would pose a threat one above their true level because adventurers have to fight them in their aquatic terrain.

I seem to remember that the school district, before being a school, was a warship created specifically to confront Leviathan on the high seas.

I don't exactly rmb this detail, but I seriously wonder what makes Leviathan can not flip the school district or just sink it from under the water. Well, about this, we can just wait until Omori showing us the fight.

Yeah, Amphisbaena doesn't move on land. From what I understand the thing is that as an aquatic monster its mobility while fully submerged is way higher than when it is just moving around in the surface of water. Kind of like Marie who was obviously nowhere near Bell new Level 4 in terms of power but while swimming outsped a running Bell.

I don't know if DM19 mentions it but in SO13 it is explained the School District was originally made to fight the Leviathan and it served an important role in it. As for the Leviathan being unable to sink it or anything like that, well who knows, but if it was made specifically to fight it it's not really rare to think they figured out a way to avoid that.

@Longhatruong, thinking about it, I think you're right.

The fact that Amphisbaena is level 5 on land and level 6 in water is inaccurate; Aisha explained it like this: it is considered level 6 by the guild but its real status is level 5, it is given a higher level due to the difficulty of facing it underwater.

What led me to consider it an amphibian was that it was able to get out of the water when it jumped to the ceiling of the 25th floor to launch itself like a kamikaze and make the floor collapse; which means that it can get out of the water but that doesn't make it an amphibian. Sorry for the inaccuracies, I read volume 14 years ago!

That being the case, the parallel you are proposing between Amphisbaena and Leviathan could well be true, if it were the case that adventurers who are not experts in maritime combat had to face it underwater, Leviathan could have a great advantage just like the one Amphisbaena has (one level). But remember that there is the Diving development ability that might well mitigate the effects of the aforementioned advantage, therefore, I would believe that based on this DA it is determined for each particular case how much disadvantage it will have in underwater fights.

There is also the fact that the combat itself could be amphibious, as was the one against Amphisbaena where the Hestia team fought from "land" while the dragon was on the surface of the water. In the case of Leviathan, it could have happened in such a way that those with the Diving AD fought in the water and those without it fought from the deck of the ship, so Leviathan's advantage would not have been constant or universal throughout the fight.

For these reasons, I believe that it should have been a level 10 in order to represent a great challenge for a team with 1 level 9 and perhaps some pseudo level 9s as well, since as I explained previously, it cannot be generalized that the monster had a 1 level boost over all its opponents.