Personally, I don't flight Marlixes in a cave. I think the best way to battle Marlix is swimming in an ocean or a deep lake while having a Respiration Helmet and Depth Strider III Boots. In this aquatic environment, you're able to engage the battle in two ways. Offense: Keep your head above the water and attack Marlix with a bow. Arrows deal much more damage to Marlix than melee weapons. Defensive: When your health or hunger gets low, dive down to restore them. While deep underwater, arrows will lose their initial velocity very quickly by the water. EDIT: As long as you stay in the water, Marlix's flaming arrows can't burn you.