expression, godan verb (-ru)
assail the enemy; attack the enemy
Fixed expression meaning to engage or attack the enemy, often used in military or historical contexts.
兵士たちは敵に掛かる準備を整えた。
The soldiers prepared to assail the enemy.
攻める is a general verb for 'to attack', while 敵に掛かる is a fixed expression specifically meaning 'to assail the enemy'.
Compound of 敵 (enemy) + に (particle) + 掛かる (to go toward, to attack). The exact historical derivation is uncertain, but it is a conventional expression for engaging the enemy.