great rival; powerful enemy; archenemy
湿気はクッキーの大敵だ。
Humidity is a great enemy of cookies.
我がチームは宿命の大敵と戦う。
Our team will fight our fated great rival.
目標達成を目指す者にとって、怠惰は大敵となる。
For those aiming to achieve their goals, laziness becomes a great enemy.