Finally managed to beat Leviathan 2. Thanks for the tips on using Ice attacks to break the shield and levitation to avoid the maelstrom (never occurred to me that levitation would help for that). Before that she was kicking my ass even though I was well past the recommended level 45, and I didn't think I would even beat it. Got Leviathan to drop too (didn't drop from the first stage), but no item. Guess I'll have to try again for the drop.
I don't know how I'm going to beat the third stage though. I just barely have a level 60 average level for my best team, and no one who can up my ice resistance. If the level 45 stage gave me trouble, I can't imagine I'll fare well on the level 65 stage.
Edit: Got the item drop on my second run of stage 2. Stage 3 will have to wait until later (out of energy), but I wonder if I should spend the next few days grinding instead, and tackle the final stage in the last couple of days of the event. With 40 energy per try, I can't really afford to go in unprepared. Too bad there is basically no way to get a fire resistant hero between now and then.