以毒攻毒
to set a thief to catch a thief
以怨报德
He is a person who bites the hand that feeds one.
以逸待劳
To wait at ease till the enemy is exhausted.
以寡敌众
以德报怨
I think it is better for you to render/return good for evil .
以其人之道还治其人之身
to pay somebody (back) in his own coin
以子之矛，攻子之盾
to turn a person’s battery against himself
以柔克刚
Willows are weak yet they bind other wood. (Soft and fair goes far.)
以和为贵
A bad compromise is better than a good lawsuit.
以卵击石
What you do is throw a straw against the wind. (run one’s head against a stone wall; Whether the pitcher strikes the stone or the stone strikes the pitcher, it is bad for the pitcher.)