That's funny. People on the forum say that the A.I doesn't do that....but its good it does that so the challenge is up! You should try the same
You can make a doomstack but you need some defenders around cities as well. In Heroes V it's superhero vs superhero....I liked the campaigns there but I never started a custom map.
Well that map I lost due to it (and basically because I've not any effective spells available to go against the Shadow Stalker unit). Nevertheless I found out where the hostile target city was and just rushstormed that in a replay. The AI had already a near-invincible full army there but I just stayed adjacent to that city and bombarded it every turn with a spell taking away 10 or 15 HP from all units, until their combat strength was in acceptable levels ^^ AI could have offensively engaged me in a single turn with a sure defeat of me but instead decided to be bombarded... anyother flaw in the AI IMO.
The campaigns in HOMMV are ok except for the rather dull dialogues, some are too easy due to your heroes abilities (esp. Necromancer) some quests can be bugged and require a restart.
I found the Scenarios much more interesting and played all of them, successfully on heroic, although usually picking Necromancer & Dungeon faction which I find the two most powerful (under certain starting heroes). I'd venture to say that it's near impossible to win some of these scenarios on heroic under a weak setup.
I've also collected many fan-made scenarios but most of them aren't that well-balanced or professional than the one coming with the game. IIRC the 2 xpacs don't add any scenarios anymore but you can play the multiplayer ones solo against the AI having an AI ally.
In some of the larger one you can actually raise multiple heroes to max level, the rushstorm-the-AI tactic doesn't work there on heroic (the AI basically sacrifices all his power to get high-tier units as fast as possible and how you survive the first engagement against these units is the #1 thing to care about if you wanna survive in the long run).
The AI is very good in monitoring the placement of your offensive forces and the time it takes to retreat back to your fortress. So if you decide to go offensively against an adjacent AI another one will surely try to capture your own home before you can be back. So a lot of time you're forced to play in a stalemate. However, the AI against themselves will take their chances and one of them will emerge as victories --> which means that, by the time you get the message that one AI got obliterated from the map, the successor now got 2 fortresses to draw units from each week, which he could possible throw against you, so your position to just play defensively will gradually result in a defeat as more time passes. Thus you are forced to take risks.
I've found that the very best strategy to counter this is to have a number of own heroes, placing them in a row so you can move back your mainarmy in a turn or 2 from your expansion-front back to your castle.
On the other hand, I would never play a game without challenge cause I need a challenge to overcome.
That's why I'm basically fine with granting the AI hidden bonuses on higher diff levels. Lots of folks out there calling that "cheating" when they don't understand the necessary function of it (or they could simply play on normal/standard levels instead...)