The poet E. E. Cummings stood for the individual human being against regimentation and standardization of any sort.
Conclusion
In standing against regimentation and standardization of any sort, Cummings stood against something essential to the work he did (standing for the individual human being).
Evidence (since / and)
Metaphor presupposes literal language, and literal language essentially involves regimentation.
Evaluate
Excuse me, waiter? Hi, I believe there's a mistake. I ordered a question 2, but this seems way too hard to read for the 2nd question in an LR section.
Okay, when we're confused we can lean on structural clues. We have to find out Conclusion, and "since" and "and" instruct our eyes that the final two claims are supporting ideas.
The conclusion has a pronoun part of it, "in doing so", so we need to import language from the first sentence in order to grasp the full claim that the Conclusion is making.
When we're confused on Sufficient Assumption, we can lean on a trick I call "Mentioned twice? That'll suffice." The basic idea is that ideas that are mentioned twice (literal language, regimentation, stood against) are pretty much taken care of, in terms of the internal logic of an argument.
Consider this "argument": If A then B. Thus, if A then D, since B presupposes C.
Without thinking too hard, we can guess that the missing link is between C and D, since they are only mentioned once, whereas A and B are each mentioned twice.
So in this Cummings argument, the stuff only mentioned once is "essential to the work he did" and "metaphor".
To actually understand and predict that link, we should first take advantage of the mini-chain of ideas the Premises are giving us in the final two claims.
Metaphor presupposes literal language. Literal language involves regimentation. Metaphor → literal language → regimentation
If you're against regimentation, as they say Cummings was, then by contrapositive, you would be against literal language and against metaphor.
The author is saying,
Goal
Thus, we want something like "metaphor was essential to the work Cummings did".