How do I complete an essay using the 'key terms' without it sounding repetitive?

The short answer here is, 'you can't'. GCSE and A-Level markers will look for continued acknowledgement of the question and so you must use the 'key words' to conclude all of your relevant answer material. It may be difficult at first but as long as your 'points of argument' are strong and developed, the repetition of key words will not make your answer seem juvenile. You must focus on a range of suitable vocabulary and valid contextual material instead of the rather 'clunky' repetition in order to maximise your marks.