the usage of に in this sentence


腕に当たっただけでこの威力 says a person just got hit. But that に doesn't make too much
sense, he never hit his hand, but he says he hit it? Or does he say he was hit
and the 当たる just functions as passive?




It is the latter, or rather the subject of 当たった is omitted. In full, it would
be

  * **それが** (自分の)腕に当たっただけでこの威力(だ/をもつ)

meaning _That has this impact just by hitting (my) arms_.

Note that the subject of だ/もつ is still missing above, more literally,
forgetting about だ/もつ, it may render as _This (much) impact by this hitting
just my arms_.




It is the latter, or rather the subject of 当たった is omitted. In full, it would
be

  * **それが** (自分の)腕に当たっただけでこの威力(だ/をもつ)

meaning _That has this impact just by hitting (my) arms_.

Note that the subject of だ/もつ is still missing above, more literally,
forgetting about だ/もつ, it may render as _This (much) impact by this hitting
just my arms_.

