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 by comparing the impact of the cannon fire to hail hitting a roof, emphasizing the strength and durability of the Merrimac with the metaphor of the "monster’s hide."