Answer: The literary device used in the given passage is an example of an "oxymoron".
Step-by-step explanation:
Oxymoron is a figure of speech that combines contradictory terms. In this case, the lines "She'll not be hit with Cupid's arrow" and "From Love's weak childish bow she lives unharmed" present the contrasting ideas of being unharmed and unaffected by Love's arrows despite the mention of Cupid, who is associated with love and attraction. Which makes the opposite meaning at the same place which creates an oxymoron.