I agree with Janet. Testing and PPC will help, but more than that your intuition helps most.
Bucket those keywords based on intent.
Essay editing, booked editing, and resume editing are slightly different so I'd make a page for each (include examples, be specific to that type of project). But when it comes to broad variations, go by the intent.
Essay correction and Essay editing should be the same page, and I'd mainly target editing as it sound more like what your product is, but use essay correction in the text. And try to vary the anchor text to the page when you link internally and externally.
When it comes to small slice of a big pie or big slice of a smaller pie, that's a call for you to make. Check out the competition on essay editing ... can you compete? Are you better than them for the searcher? How? Make that apparent and push for it with marketing. If you are the same ... but can own "essay correction" (the SERPs are not as competitive) ... go after that first.
Hope this helps!