“shooting fish in a barrel”别理解成“在桶里抓鱼”，引申为小事一桩；很容易办到的事；没有挑战的事。

【例句】
What do you mean you can't do it? It'll be like shooting fish in a barrel!
你说干不了是什么意思？这不是小事一桩嘛！

Jane's a good mechanic. Changing a tire is like shooting fish in a barrel, for her.
Jane是个很好的机修工，换轮胎对她而言是件没有挑战的事情。