Leviathan can be somewhat tricky without using her weakness, however she's also one of the most patterned bosses in the game. She uses all of her attacks in a certain order, and if you can figure out how she reacts to your own movements, you can figure out what shell use and where she'll use it pretty easily. Do be careful of her swift movements though. Accidentally coming into contact with her is probably the way you'll take the most damage.
