Here "went down ... to Sheol" is an idiom that means to die. Alternate translation: "those trees of Lebanon also died and went down to Sheol with the cedar" (See: [[rc://en/ta/man/translate/figs-idiom]])
Here "sword" represents enemies who fought them in battle. This can be stated in active form. Alternate translation: "whom enemies had killed with swords" or "who had died in battle" (See: [[rc://en/ta/man/translate/figs-activepassive]] and [[rc://en/ta/man/translate/figs-metonymy]])
"These trees of Lebanon were its strong arm." The phrase "strong arm" represents "power." Alternate translation: "These trees of Lebanon were the power of the cedar" or "These trees strengthened the cedar" (See: [[rc://en/ta/man/translate/figs-metonymy]])