Final answer:
The phrase in question uses the literary device of personification by attributing human actions to the abstract concept of life. It is not a metaphor or simile, which instead make direct and indirect comparisons, respectively.
Step-by-step explanation:
The phrase 'Life's a poor slayer who struts and frets' is an example of personification. Personification is a literary device where human qualities are given to animals, objects, or ideas. In the given example, the abstract concept of 'life' is personified as someone who can 'strut' and 'fret,' characteristics typically associated with a human being.
It is important to distinguish between similar literary devices. Metaphors and similes are also commonly used for comparison. A metaphor makes a direct comparison by stating something is another thing, such as ‘Hope is the thing with feathers’. On the contrary, a simile uses 'like' or 'as' to make a comparison, for example, ‘She is as sly as a fox’. Since the phrase in question doesn't use 'like' or 'as', nor does it say life 'is' another thing directly, it does not fit the definitions of simile or metaphor. Instead, it attributes human actions to life, which fits the definition of personification.