Let's try and math the HP problem some more. This is not definitive, but it's an approach I'm trying to find out a reasonable cap.
I'd like to have a reason why 30HP, or 50, or whatever number. Here's to finding out why a given number would make a cap.
In my cost formula, as units increase in HP the attack cap reduces. This was intentional, to make it so that high-HP units are not good at offense. High-HP units are good at, well, HP however

so let's see what that implies.
Returning to the LOLCatiusha vs. Diwigible stack. LOLCatiusha*10 and SniperCat together have 200 Attack and 105 HP. The Diwigible stacks have 186 Attack (which against the LOLCatiushas becomes 116 because of Defense) and 150HP. Yes, more HP for the Diwigible stack, after all the LOLCatiushas spend points on Defense and also a larger chunk of each LOLCatiusha cost goes into attack.
So
the LOLCatiushas need to have a random roll of 75% or better to croak the entirety of the Diwigible stack.
The Diwigible stack needs 90.5% or better to completely wipe out the LOLCatiusha stack. Without knowing the probability function it's difficult to tell just how much worse this is, but I'd say it's, worst case, "even". The LOLCatiushas have a better chance to wipe out the Diwigible stack.
What about the Diwigible on its own? It costs 422, from which I can get 7 LOLCatiushas with 2 points left spare. So I have a Diwigible with 100Hits and (effective) 54Attack vs. 7LOLCatiushas which together have 70HP and 133 Attack. The Diwigible cannot croak all LOLCatiushas but
the LOLCatiushas can croak the Diwigible if they roll 75.2% or better. Alone, the Diwigible is way worse in the offense department.
Now, this was just one case. A more general approach is to say something like this: a unit has X hits (and we impose 0Def and 5Move to make it Diwigible-like). Assume also that it carries the cheapest, best-offensive stack it can carry. Then, compare this stack to a stack of some other smaller units; both stacks should cost the same. What odds do each of the stacks have to wipe-out the other?
I
think that since the attack cap gets vicious as Hits increase, similar comparisons will reveal similar results as we increase the Hits.
If a unit has X hits, then it can carry X/2 Hits of small units. Small units can sum attack that is twice their hit value, so our X hits unit can make a stack with X+Attack Cap(X) offense. The ratio of this number to X will get worse as X increases, since Attack Cap(X) limits attack sharply for high HP units.
OTOH, as a unit grows larger it can carry more very slow and cheap units, so maybe a 200HP Diwigible with enough Orc Archers would fare better against its weight in LOLCatiushas.
Then, there's the amount of attack you need to field against a high HP unit to kill it. I'll try and "math" that too somehow, but it's fuzzier.
In principle, any HP amount can be killed. The thing with High HP units is that they will kill
something, even on a bad roll. However, a stack of small units needs to be at least a certain size to guarantee downing the Diwigible.
The whole point of this is lost if you keep it a secret.