Imagery refers to the descriptions used in writing and the pictures they create in the mind, appealing to one or more of the five senses. Writers use imagery to convey a picture without saying directly what the image is.
Here, the writer uses war/fighting imagery, with healthy foods (superfoods) competing against infection, cholesterol, cancer cells, calories, ageing, heart disease, etc. War-related words are used throughout to create vivid pictures in the mind, e.g. 'knock out', 'crush', 'wipe out' (lines 1-2).
Read the article again and find words related to war or fighting. You'll find that these words make the article more interesting and lively than if it was simply a straight-forward report on health and nutrition.
Try using such techniques in your writing to make it come to life.
Answers: heroes (l. 2 & 42), action figures (l. 5-6), superpower (l. 16), pack a ... punch (l. 19-20), busting (l. 20), fighters (l. 23 & 31), foe (l. 28)