Levi can be a real challenge to less experienced players, but he's very killable once you figure out his strengths and limitations.
Levi is incredibly good at wiping out single models in a given activation (as you saw) and unless you have the resources to kill both Hollow waifs in a single turn he's also a challenge to actually kill. Conversely, his most effective spell is ranged, meaning he can't use it in melee and will have to randomise if his target's in melee. His damage output is also strongly tied to his wounds - if you can take him down a few wounds there's a good chance he won't be able to do as much damage as he'd normally hope to. So if I want to keep Levi out of the fight for a turn, I'll run a few of my more expendible models into melee with him and try to keep the rest of my crew in melee themselves.
Levi is also quite slow and since he summons at the end of turn to a Hollow waif, it's easy to predict where he'll be in the next turn, especially if he's taken Rusty Alyce. You can also dictate where he pops up by killing one of the Waifs yourself. He'll live, but it generally means he'll have to resummon to the other waif, who's often going to be in a safer position where Levi will be less capable of getting into the fray.
Steampunks - if you can kill even one of the four, they lose the ability to become the desolation engine. Failing that, if your opponent runs a deso engine up to your crew in the first turn then it means you're free to concentrate on the engine with minimal retaliation from your opponents crew.
Looking at Ironknight's list, it seems pretty standard. I'm guessing Molly is there to boost Levi's movement but aside from the potential desolation engine there's not truely hard-hitting melee minions. Rusty is merely okay in melee (she really wants to be shooting her gun) and while Levi can do a butt-load of damage with his fist, it's preferable to him slinging ranged spells at you. So here's my advice;
1. Avoid Levi's spells, either by keeping outside his LoS, distracting him with chaff or staying in melee with the rest of his crew. Fortunately, from what I gather Flesh constructs can count as "chaff" in a Mcmourning crew.
2. Remember that if Levi can draw a bead on a model, that model is likely to die.
3. If you can take 3+ wounds off Levi before he activates, he'll generally be less effective that turn.
4. Set out to kill any Hollow waifs that you'd rather Levi not summon to. Even if you can't kill both, this will let you dictate where Levi is able to project his power.
5. Threaten the Hollow Waifs with something like Necropunks. It will force him to hold them back and by extension, prevent Levi from getting too far forward.
6. Don't let the Steampunks reach critical mass if you can avoid it. You just need to kill the forth and/or prevent them from getting into BtB. Fortunately they're not hard to kill, and if Levi is resummoning them from scrap it means he's not using those AP to kill models.
That's all I can think of for the moment. I hope that helps.