expression
don't let your guard down; carelessness is the great enemy
A yojijukugo proverb warning that letting your guard down is the greatest danger; used as a standalone admonition or in cautionary contexts.
油断大敵、最後まで気を抜くな。
Don't let your guard down—stay focused until the very end.
試
Until the exam is over, carelessness is the great enemy.