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?
1 Answer
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.