The phrase "like shooting fish a barrel" is a commonly used idiom that suggests something is incredibly easy or simple to do. However, if you are looking for alternative phrases with similar meanings, you might consider alternative idioms such as "a piece of cake," "child's play," "a walk in the park," "a cakewalk," or "a breeze." Each of these phrases is used to describe something that is easy or effortless, much like shooting fish in a barrel. These are just a few examples of some of the alternative phrases you could use when wanting to communicate that something is a simple or easy task.