It's more Beo being dumb rushing straight at him like a moron, honestly. Although Soujuurou's strong by average human standards (supposedly thanks to growing apart from civilization), he broke both arms in the process and the elemental would wreak his ass afterwards.
But as said above, psychological breakdown (becoming aware of his lack of invulnerability) and thus learning fear - becoming "useless" (imperfect) according to Touko.
If you think about it, his ego was pretty up there after traveling all over the world (doing all kind of crazy stuff with Touko, including making a DAA run away), being immune to magecraft, etc... And then he gets his heart damaged by a puny human with no special power.