Hekayat 24 from Chapter 8 of Golestan of Saadi

Strike the head of the snake with the enemy's hand, for it will not be devoid of one of the two benefits: If this one prevails, you have killed the snake, and if that one prevails, you are rid of the enemy.

Do not feel safe from a weak enemy on the battlefield,

For they will overcome you like extracting the marrow from a lion’s brain once the heart is detached from the body.

سر ِ مار به دست ِ دشمن بکوب که از اِحدَی الحُسنَیَین خالی نباشد: اگر این غالب آمد، مار کُشتی و گر آن، از دشمن رَستی.

به روز ِ معرکه ایمِن مشو ز خصم ِ ضعیف

که مغز ِ شیر برآرد چو دل ز جان برداشت