Not much to say other than use buff potions (the best ones or the ones you can currently make) and git gud. It even took me around 17 - 20 tries before I could kill the Leviathan (and this was on EXPERT MODE, mind you) and even then, half of the time I was studying the Leviathan's patterns instead of battling her directly as to get a good grip on gaining the upper hand for future battles.
If looked at carefully, the Leviathan battle is the Twins battle on steroids, Retinazer going pure bullet hell mode with Spazmatism becoming the second largest boss and going pure tank mode.
If you're still stuck and need a useful tip for later encounters, here's one grabbed straight from the boss' description straight from the OP: