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
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.