User blog comment:Onida DSTY/Strongest Overlord Characters/@comment-36770034-20180902191924/@comment-34598988-20180915231130

it was stated that the monsters around a dungeon would indicate the levels of the monsters inside the dungeon there were level 85 monsters outside nazarick with level 90 monsters inside it was also stated to be a level 90 dungeon so natrualy it would have a level 90 boss momonga stated that all the member of nines own- or rather ainz ooal gown were in the 90s and finally asura's strongest summon was a level 90 primal star elemental his level is never clearly stated but its not hard to figure out that hes in the 90s range even if you want to stretch it and say asura could have been level 100 my point still stands that a level 100 player would not be ableto solo him

plus this is pretty common knowlege if you look at most MMO games a player would not be able to fight an open world or raid boss of an equal level alone for example eden eternal if you were to attack the level 90 world bosses solo at level 100 you'd be lucky to survive 20 seconds it takes 10-15 level 100 players to take a level 90 world boss (unless you spend months fortifying gear)