noun, noun or participle which takes 'suru'
raining down (arrows, bullets, etc.); showering upon
Literary or classical expression for a concentrated volley of projectiles, likened to a downpour. In modern Japanese, 雨あられと降り注ぐ or 集中砲火 are more common.
敵陣に矢が雨注した。
Arrows rained down upon the enemy camp.
「雨注」は、矢や弾丸が雨のよう
雨注 is an old-fashioned expression describing arrows or bullets falling like rain.