I just did the maths for a (rumoured) Fists Leviathan firing at an IH repulsor. 20 shots, hitting on 2s with rerolls, wounding on 5s with rerolls of 1s, 5++ saves and 6+++, with hits causing 2 damage. The result was 8.4 damage.
This is not a hard counter. A Leviathan is out-ranged by a repulsor and a Fists one presumably won't have the Ironstone or 6+++. An IH leviathan obviously does half as much damage in a mirror match vs other iron hands - doing less damage per turn than Feirros can heal.
A Fists Deredeo actually does 6.3 damage to an IH repulsor, for significantly less cost than the Leviathan and with a 5++ aura and 36" range. That might be a better way to go.
Stormhawks are another interesting option. Repulsors have -2 to hit them. Equip them with heavy bolters, stormcannons and twin assault cannons and try to chew through things with 1D weapons. They're clearly better as Iron Hands than any other chapter, obviously, because they're vehicles and lack potms.
But to be clear, nothing actually works. The repulsors outshoot everything, without exception, and assaulting them is suicidal. I think only some version of mortal wound spam, like IF devastator centurions, stands a meaningful chance. And those HLDs are a massive threat to the centurions.
I've been looking at Raven Guard using the master of ambush trait to get assault centurions near the executioners. It stands a chance if you go first, and if your opponent has never played 40k before and fails to screen his repulsors. Those aren't factors I feel I can rely on. The list also dies on its bottom against Eldar planes.
My proposed 2k list is as follows:
Primaris Captain with stalker rifle and Iron Stone. Make a chapter master.
Primaris Lieutenant with stalker rifle. Maybe give this guy a relic gun or something, if you like.
3x5 intercessors with whatever.
3 HLC executioners with icarus pods
3 Storm Hawks with heavy bolters and icarus cannon.
I think the storm hawks are pretty horrible. They each have 21 shots with +1 to hit anything that flies and rerolls of 1s, all at S5+ and -2 ap. So they can mow down any troops or mortar squads you might find skulking on objectives, and maybe assassinate the odd character too. The whole list fires something like 200 shots per turn if everything is in range.