Note that one more situation where Leviathan shines is against Darksteel Colossus. If you get a swing in with Leviathan, then they Tinker for Colossus, you can unblockably hit two more times and race. A Colossus in that situation would create a stalemate. LSV actually won a game because of this situation as you can read in his most recent report. This has also happened to me at least one or two times in testing.
this happened to me as well last night, i won with the leviathan because his tinker was 1 turn late and since i was running the leviathan there was no standoff, he just lost
on a side note, because of leviathan i have started running hurkyl's recall in place of echoing truth since i see many mirror matches in my meta