他不擅长于棒球短打.

He is no good at Bunting the Baseball.

他不擅长于棒球短打。

He is no good AT Bunting the Baseball.

“我不擅长于…”应该说：

I am not be good at.

如果你想成为一个好的产品经理，那你得擅长于做你不擅长的事情 。

If you want to be a good product manager, be comfortable being uncomfortable .

要我观鸟，我不在行；要我追踪青蛙，我也不擅长于此。

I found that I was lousy at bird watching and couldn't track frog calls either.

据我所知，我的外婆并不做圣诞小甜饼，因为她不擅长于精工细做。

As far as I know, her mother didn't make them; she wasn't the type for delicate anyway.

今天，我们有一种称之为“色情短信”的东西，我是这方面的专家。我可不擅长于发这个。

Today we have sexting, and I am a sexting expert. Not an expert sexter.