C'mon, I believe in you! :) If you can take on Momentus, you can take on Marlix. Just bring a decent bow. If you can, fight him from on top of a mountain or hill. That way when his Guardians spawn, you can just shoot them down the mountainside. :P Always remember to take care of the Guardians first before going back at the Marlix. Fire Resistance is a good idea as well. :)