I honestly don't understand what you're arguing at this point. If Yu has access to Beelzebub, a Persona that cannot be fused until well into the late-game, then he must either be as powerful as his late-game key (which is unlikely, given how he struggles with Shadow Naoto), or the depiction of Shadow Naoto taking hits from Beelzebub is an outlier that shouldn't be used for scaling.
You are under the impression I am arguing Beelzebub is superior to Shadow Naoto simply because the former's level is higher than the other's. However, I am not employing this line of reasoning in the slightest. What I am doing is using the levels of Persona and enemies in order to determine approximately when in the game Yu would have access to a given Persona. Level 80+ enemies do not start appearing in the game until the very end of Yomotsu Hirasaka. If I were using level scaling, which you seem to believe I am, I would use this as evidence to suggest the various shadows fought in the dungeon scale to late-game Yu, which is clearly not what I'm arguing. My point is that Beelzebub, as a level 81 Persona, is intended by the developers of the game to be acquired sometime during the exploration of Yomotsu Hirasaka, which obviously occurs well after the Ameno-Sagiri fight. Beelzebub isn't stronger than Ameno-Sagiri because 81 is a higher number than 75. Beelzebub is stronger than Ameno-Sagiri because, at level 81, Yu would be at a point in the game that occurs after he had already defeated Ameno-Sagiri.