Arch Enemy
Rival, Enemy
noun
Your big enemy is your worst enemy. Your arch enemy.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
ワニカニの大敵はだれだと思いますか？
Who do you think is Wanikani's rival?
オレたちの大敵が、ついに敗北を認めたぞ。
Our arch enemy finally acknowledged their defeat.
分かっていると思うけど、油断大敵だよ。「サルも木から落ちる」って言うだろう？
As you all know, carelessness is man's greatest enemy. People say "even monkeys fall from trees," right?