Your essay needs to be consistent in tone (formal or informal, familiar or detached, straightforward or ironic). Don’t jump between scholarly and familiar vocabulary and style. To achieve consistency in tone, your diction (word choice) should be consistent in level.
Academic writing generally requires a formal, detached, straightforward tone. Avoid informal language or slang, such as “get together,” “a couple of,” and “go crazy.” Avoid complex terminology or theoretical jargon unless your essay specifically belongs to the school of thought that requires such terminology/jargon.
Substituting synonyms that you find in a thesaurus tends to produce sudden “spikes” in the level of your diction. You should have recourse to a thesaurus only if you can’t remember a word or if you find yourself repeating a word and want to find a synonym to vary your vocabulary. Any word you find in a thesaurus should be checked in a good dictionary. Make sure that you really want to invoke the precise denotation/connotation of that particular word.
Archaic words usually sound affected, particularly if the rest of the sentence is standard or colloquial in tone: “As was his wont, he bought his matinal coffee at the SUB.” Why not just say, “As usual, he bought his morning coffee at the SUB”? Archaisms are hard to resist if you love Medieval and Renaissance literature, but remember that you are a post-modern person writing about Renaissance writers, not a Renaissance writer yourself.