Viability question here. TL;DR is below the large quote.
I just conducted a test run with Nasus running for Leviathan in a solo q game. I used a lvl 10 smurf that had 17 wins (18 after this game) and 12 losses, went duo a few times with a friend before.
Twitch, (expert in the art of ganking and knows how to play in general)
Annie, (uh... her first item was Tiamat, SOMEONE HELP THIS GIRL!!)
Blitzcrank, (decent, knows how to Rocket Grab, sometimes ran head-first into trouble)
Janna (when she got first-blooded before minions spawned, she complained I couldn't do shyt. Was not a team player until late game)
Shen, (knows his turrets, hard to gauge his skill level seeing how Shen is new)
Poppy, (knows her turrets, knows how to ult, DOES NOT know how to charge)
Annie, (decent, did WAY better than our Annie, lol)
Ryze, (thank jebus he didn't get Mejai's, but otherwise decent)
Veigar (decent, knows how not to die at least)
My item progression (not ideal unfortunately):
--Doran's Shield (1) + HP potion
--Shoes (2) + Sapphire Crystal (3)
--Sheen (3) + Ninja Tabi (2)
--Aegis of the Legion (4)
--Banshee's Veil (6) (didn't help too much lol)
--Sold Doran's, got Vampiric Scepter (1)
Leviathan max stacks obtained: 16
Leviathan stacks finished: 13
Summoner Spells: Exhaust, Teleport
--Our Annie and Janna died a few times. Since Janna dumped me early, I had to turret hug for a while against Shen + Poppy. Not fun.
--Twitch started ganking. It was pretty sweet, but we couldn't get too much with tower pushing.
--Lost top and bot tower.
--What turned out to be small skirmishes soon turned into team fights. We just kinda lumped together. Naturally I ran into the fray and ulted, got kills. Janna in the meantime was out on her own lane or something.
--We were eventually able to push down towers, and my continual presence (and Blitzcrank eating up all the spells thus dying) turned out to be quite a problem. Dragoned a few times. Some people split off to lane top/bot.
--Rushed down mid and went straight up against their base turret. By this time the enemy got smarter, but not smart enough. I died, but in turn Blitz and Twitch picked off people one by one with help from Annie and Janna. Never quite an ace though.
--Purple team surrendered 4 to 1.
--My session closed immediately. DAM U RIOT!! =(
--Need... mana... badly... dying...... of oom.... syndrome....
--I got a triple kill at one point, and the announcer actually announced it! Fk yeah!! But I didn't have Leviathan at that point. FML!
--I was somehow doing ungodly damage without Siphon Strike or the ult. Or it may be I'm just made of pure awesome. And I did get to lvl 18 the fastest.
--Enemy team was smart enough to get red/blue rune. We kinda didn't get runes, but instead stole it from their corpses. What's up with that?
--I need to look at my damage while I'm ulting some day.
--A good number of those team fights ended up with me being low on HP. Kinda sucks tanking, but I'll survive thanks to lifesteal.
It seems easy to charge up Leviathan, given that Nasus has an AoE and deaths are likely to ensue when he plays his role correctly, thus earning you assists (just as good as kills). Unfortunately, due to how battles might go, expect to die a few times, because some people freak out and go into "OMG KILL NASUS" mode or something, or the next-best-carry ends up killing you, or you turret dived. Point being, deaths are fairly inevitable. It's a question of how often you'll die.
Now I realize that was a fairly low elo game, but I do have experience and expectations of mid elo. Hell, I won a game with Nasus on my main account when doing it for the first time (3/2/5) (no Levi). I'm thinking if the teams are at least fairly even and you don't die for stupid reasons, you should be able to maintain 7-13 stacks, which is an okay investment I guess.
So, excluding high elo games (where deaths don't happen enough), think going Leviathan is a-okay for Nasus?