Final answer:
The phrase in question is a metaphor that compares Christina's shoes to causing her discomfort, not meant to be taken literally.
Step-by-step explanation:
The phrase 'Christina’s high-heeled shoes were killing her feet' is an example of a metaphor. A metaphor is a figure of speech that makes a direct comparison between two unlike things without using the words 'like' or 'as.' In this instance, the phrase suggests that the shoes are causing Christina discomfort as if they were capable of causing her physical harm, which is not meant to be taken literally but rather to emphasize the level of discomfort.