It was said that there were 2 Level 7 and how would he be able to stand against Ottar if he/she wasn’t a Level 7? The only one that is able to superpass the gap between levels is Bell. So is not a making up fact.

The same way other adventurers fight higher level opponents to level up, using techniques, skills, development abilities, etc. Bell just has an easier time because of his op stat boost, there are plenty of examples of adventurers fighting higher level monsters

Claiming something that hasn't been explicitly mentioned (barring obvious implications) is still speculation, not fact

I don't think its a good example to use monsters as a comparison when you fight a person, if we are talking about 1v1. Defeating a monster equal to lvl 3 is a completely different matters with defeating a lvl 3 person. Just like you said, adventurers can use techniques, skills, etc to outplay the opponent while monster might do the same but ofc with limitation. I mean minotour can't use magic right? unless its not a normal monster. Other than that yea its not a fact if he's a lvl 7 or not but If this knight of knight fight with his own skill alone, either Ottar was so dumb that he doesnt have his own technique, skills, development abilities etc, that the only thing that he has is an incridible strength(umm like monsters?), or he's a broken chara like Bell.

I know, the point I was making is that it isn't impossible for low levels to fight higher level opponents

We don't know anything about the "Knight of Knights" so speculating about him is kind of pointless

Yes, but still as the wiki says Ottar is one of the only 2 Level 7, so if the knight of knights it is said that is the only one able to fight against him then it just make sense that he is the other Level 7

Well,the game isn't canon,most of it is just for sake of fun,it does not coincide with light novel.

And I have a doubt,is Mia grand really only level 6? It wasn't stated or confirmed by Author anywhere,I think. Maybe she retired after she reached level 7. What do you think?

It was mentioned somewhere that she is a lvl 6. And even if this info was false I doubt she is a lvl 7. If she was then it would be known. On another side if she hid the fact that she is a lvl 7 then the world would be sure that there is only one lvl 7(ottar). So I am quite sure everyone knows who exacly is the other lvl 7 it just wasn't mentioned in the series yet. 

It's mentioned near the end of Episode Ryuu

most probably it would be from the Zeus Familia aka Bell's [adopted]grandfather's familia