Kunihiko [topic-は] first [possessive-の] one-shot [with-で] target [object-を] shoot-hit-[past-た]
撃ち当てる is another compound verb — 撃つ (to shoot) + 当てる (to hit/strike) = 'to shoot and hit' = 'to hit the target'. 一発 uses the counter 発 for rounds/shots. The で marks the shot as the means by which the target was hit.