pay someone back in their own coin

retaliate by similar behaviour

以其人之道，还治其人之身

to coin a phrase

said ironically when introducing a banal remark or cliché

套用一种说法

I had to find out the hard way—to coin a phrase.

套句老话说，我是费了九牛二虎之力才搞清楚的。

the other side of the coin

One of two differing or opposing views or sides.

反面，反面意见：两个不同的或相反的观点和方面之一