关公门前耍大刀 There's no need to teach a fish to swim.
严以责己宽以待人 to be severe with oneself and lenient with others
铁石心肠 a heart of steel
听天由命 to be guided by destiny
骄者必败 Pride goes before a fall.
小Car笔记：
lenient：宽大的, 仁慈的 【例句】①Lenient parents; lenient rules.仁慈的父母; 宽松的规章制度 ②In your case, we are prepared to be lenient.根据你的情况，我们拟予从宽处理