Your only real bet is having a bigger army in a lot of cases, but that army doesn't have to just come from a dark empire.

Orcs are great, especially when aided by a dark empire, as with a shipyard that can allow Orcish Industry to basically just generate itself, leading to each fortress having a high chance of having a 80~ army strength army. Also keep in mind, if you're building orcish fortresses you want them to have as many connection points as possible, as they create an army based on neighbouring industry strength.

Elves are great for armies as well- Both as an actual source of a dark empire, or alternatively as a distraction/meat grinder force, since you can easily turn the empires of the world against the elves without actually turning the elves into a dark empire, which'll then allow your dark empire who won't get distracted by elves to be the only real standing armies on the map. And elves always have a commander in each of their armies, meaning they almost always trade favourably.

The First Daughter can provide quite a large army that generates somewhat absurd amounts of arbitrary threat, and serves as a great distraction for the worlds armies, while also allowing you to slowly destroy cities with her aura effect, which as with all city destruction methods permanently removes an army.

If you have the DLC, the Shoggoth serves as a combination between the First Daughter and the elf method, spawning a good number of large armies which can each individually distract most of the worlds armies while also tending to trade favourably simply due to the number of armies.

Other than that, if you can reduce a cities population, easiest to do with a plague, that'll translate to a rough army size cap reduction from that city, and obviously if it manages to wipe a city out that stops the army from respawning until the city is rebuilt, which is very effective.

Commanders can also help a lot- Choosing an empire that has a lot of high command heros will ensure some of those heros, regardless of their personal enshadowment, end up commanding your armies, which lets your armies trade favourably. You can also get an Orc Warlord up to insanely high command pretty easily, since its all they really need to spec into for orc management, and then they can serve as good commanders even if they aren't actually part of your dark empire.

after a few tries i manged to fluke a win in the high 400's turns. cheesed it out by waiting till the last second to pull the trigger on the dark empire activation and won with a huge score but it didnt feel like a very satisfying way to win tbh. i was annoyed that the previous time i tried the empire thing they just got annihilated before i could even really do anything. i think theres a bit too much stuff working against you for my liking

My first win was with a dark empire and I think the key thing is to spread enough shadow across the prospective empire to not start a civil war. No civil war means your armies are all full strength, and you can mop up small neighbours to add them to the empire. If there are nearby rivals with large armies, you do want to find a way to weaken them though. Orc hordes, civil war, shoggoth, drop something on their doorstep to keep their military busy and you should have plenty of time to stabilise a militarily strong dark empire before the good guys can turn their attention to you.

When you form something like a dark empire or theocracy with humans, you need to make sure the majority of the military strength in the nation you're trying to claim is in areas you've gotten to 100% shadow/faith in. Human nations will fall into civil war if all of the city rulers don't agree with becoming a dark empire / theocracy. For other races like elves, they will always follow their ruler and don't require every settlement to be shadowed.

One of the more confusing aspects of this game is that all cities are not born equal. Capitals get trade bonuses to their prosperity, and the cities with the most population and prosperity will have the biggest armies. Aiming for cities and nations in the "green belt" of the map will give the biggest returns for effort spent when making a dark empire or theocracy.

I've had a few good Dark empire runs from just dragging the game out until the Alliance collapses into a civil war after which I just start wiping their cities while they are busy to actually having overwhelming military superiority because a large elf state merged into willingly it after being enshadowed, providing all its troops. Sometimes it really doesnt do much though, the way this game works is that the area you setting up shop in is likely to be more troubled and weak than the good guys.
Maybe if i was a better player i'd have time to screw the big important enemy empires with manual vendeta/civil war cycles, but i usually dont have resources (agents/turns) to do that while progressing the game plan.
At the end of the day I dont think empire really needs to be combat capable or stable by itself, it's just another disposable tool + a way to keep the good guys busy while you reach the god's endgame.

It's a good idea to plan ahead about what nation we'll be trying to turn into a dark empire. Spreading plagues and madness around what will soon be our territory is self destructive unless going for an insanity victory. We get more points from population in the dark empire than we do from killing people, so damaging our own dark empire will actually lose us victory points in the long run. Save the madness and plagues for parts of the world we're not planning to use for our empire unless aiming for a specific victory type.