The lines that use figurative language to describe the strength of the attacking ship, the Merrimac, are:
“As hail rebounds from a roof of slate, / Rebounds our heavier hail / From each iron scale / Of the monster’s hide.”
This passage uses simile and metaphor to illustrate the power and resilience of the ships in battle. The comparison to hail and the description of the Merrimac as a "monster" suggests its formidable nature.