At first I hated Arthur as a character, and I still do on some levels. But his power level being directly tied to people playing into his delusion is one of the funniest powerups I've ever seen in a Shounen manga. Great job Ookubo.
He managed defend against an enemy all the while not realizing he fought with his non-dominant hand. And for long enough for his pea brain to realize it and switch to his dominant arm and fell the enemy in one strike.