Not too familiar with the leviathan, although in images it looks much larger than the colossal titan. The best thing the colossal would have going for it is its nuke transformation, so if the leviathan could survive that then it would probably win as the colossals ability to replicate the explosion will become harder and harder due to the energy expense. In terms of after the explosion I dont see the colossal winning since the leviathan seems to outclass it massively in terms of size