Just tried Leviathan on normal as RDM/NIN, and he wasn't much harder than Shiva. I was interrupting a good number of his Spring Waters with Dia, but I'd have killed even without; they were only healing about 4000-6000.
He was harder because I couldn't land Addle II on him due to his element, so some of his Grand Falls and Tidal Waves were getting through my MEVA set, to the tune of 400-900 damage.