The author tends to confer capabilities to bell and added characters that allow the story to 'work' even if bell/hestia familia are lower level. haruhime and welf swords are examples, as well as bell's argonaut and ox slayer.
I suspect the new magic against charm is another one. since freya is defeated, this suggests that at least one, if not both, of the big remaining bosses have charm capability.
Corrupted spirit is definitely a strong possibility, but i suspect oebd also has a way to mess with opponents' heads. that could explain why level 8 and 9 teams were killed if they cannot display their full strength. and it would be another way how bell may win despite not achieving level 10 by the end of the year. you could have argonaut + bonus from enemy trying to charm that essentially allow him to launch attacks at 2+ levels above his level. at current level 5, this would be level 7+ equivalent attack. as a sanity check, he was able to hurt ottar at peak 7 while at early 5.