If you're so good,you do it意为“既然你这么厉害，那你来呀”，还可以用反问句“If you're so good,why don't you do it?”（既然你这么厉害你怎么不去做？）这两个表达异曲同工，都带有一丝挑衅的意味。

【例句】
He played so badly, I can do it better than him.
他打得太槽糕了，我可以比他打得好。

If you're so good, why don't you do it?
既然你这么厉害你怎么不去打？