Hit
Strike, Blow
noun, する verb
An assault hit is a hit or strike or blow. Not a friendly hit, that's for sure.
おうだ
The reading is the on'yomi readings of the kanji.
突然、背後から知らない男に後頭部を殴打されました。
Suddenly, I was struck on the back of the head from behind by some random guy I didn’t know.