Hi there. I can't say I'm a huge Disciples fan and as I've started playing long after its glory days, nevertheless I did play the games quite a bit and I consider myself something of a good player too. At least as far as Disciples 2 goes, first game though...
A little background to make you understand how I feel:


A few days ago I finished the Mountain Clans saga on Very Hard difficulty as a Guild Master (no HP regen SUCKS for Dwarves but some careful planning and being buddies with the Emire all the time made it doable and not at all frustrating).
Actually, aside from the very first mission, it was rather easy. Even in the last mission the enemy was docile and rarely used magic.

I've completed the Empire saga twice and the last mission is when the game becomes, uh, interesting. After passing a certain checkpoint (can't tell exactly where, it was a long time, but there's a Dragon camp nearby) the AI enemy goes f*cking postal on you and spams highest level offensive magic on you all the time. Since the goal of this map is too transform most of the map I just decided to do it the cheap way. I turned my main party invisible (saved a Sanctuera scroll from the previous level) and went straight for the Capital through the entire map. I don't see how I could have done it otherwise. My hero was a mage so he wasn't exactly mobile and it was impossible to get to enemy cities and conquer them without getting rekt by overworld spells.

Then just today I rage-quit doing the Undead Hordes saga. I've heard it's the hardest saga so I expected some difficulties, but man, what was THAT? The third mission itself is already crazy. You're pitted against two factions who are allianced and really DO NOT mind sending everything against you. Just like above, at one time I gave up conquering the map bit by bit and went straight for the capital (and it wasn't easy too). And then, in the last mission I just quit. The Clans either spam the Stone Ancestor who hits for 150 DMG, or they cast the Vengeance of Ymir for a nasty 100, and then they casually send some heroes on you. Those heroes only tend to be level 7-8 and also bring a nice pet Dragon with themselves.

So, to all the veterans of this game, what's the trick for this mission (or some other general stuff to "outsmart" the AI in any scenario)?
Hide in the Capital and let them come? Try to get another Sanctuera scroll and just go straight for the goal? Or maybe not play Very Hard at all? I love a challenge and want to get the most of the game (I also tend to play Iron Man AKA no reloads) but if the difficulty is just cheap, I might lower it next time.

It's been a while since I've played through, so I can't give specific scenario advice that much, but a major weakness of the AI that can be exploited is to deny them resources as soon and as aggressively as possible. If you can get rod planters near their home base, stick rods on all their gold and mana mines, and in general keep them off of any resource they can use as best as you're able. Once an AI's crippled this way, they'll spend their gold/mana on low level spells and low level parties instead of saving up, and an AI with no active party will always build a combat party first rather than a rod planting one, so if you've rodded all their resources you can leave a party just strong enough to handle greenhorns in their home territory to kill any new party they send out. They never get as far as removing your rods from their mines that way. From there you can mostly focus on whatever the actual goal of the quest is without the AI kingdoms being more than an occasional nuisance as long as you keep them from recovering.

Obviously some layouts make this easier than others, but sticking rods on resources they can use, even if it's mana you don't have spells for, can be powerful even so. Remember, to take something you've claimed this way, they have to first get their rod planter there, spend a turn to break your rod, and only then can they plant their own.

Oh yes, and for Undead specifically, if enemy spellcasting is being a nuisance, one thing to do is for upgrade path, go Templar. Something that's not made too clear regarding the "ward" powers is that not only do they block the first attack by that element in battle, they block ALL spell damage using that element in the overworld. So a Templar/Dark Lord can never die to earth, air, fire or water spells overland. I am fairly certain that this also applies to parties that have been warded with a spell, and heroes that are carrying a tome (something to keep in mind if you're debating the uselessness of the tome skill, even if it is generally worse than banners or artifacts).

For a more tangential tip, I'm not sure if this is an exploit exactly (it certainly seems to have far better success than the stated percent rate) but if you see some fancy, powerful item in a shop that one of your heroes can use, steal it rather than buying. It usually works and even if you lose a thief or two you've spent less gold than if you bought the item.

Hey there, thanks for the tips. Now, most of that is familiar to me, as I've said I played other campaigns on V. Hard (not to mention completing the second game several times too) and I'm aware of stuff like stealing, Wards (if you have lots of mana you can spam elemental Wards which is helpful). Are you exactly sure about claiming AI resources works as inteded? As far as I know, enemies have infinite resources on max difficulty, at least in the main campaigns.

I'm not 100% sure regarding the highest difficulties, but I don't think they ever have truly infinite resources. It's more like resource regeneration where they're treated as if they have some bonus mines, as far as I've been able to tell. I think their units also get some bonus XP per turn when hanging out in their capital or maybe just for existing.

I always rush to upgrade all the units in the capital and gain as many resources as i can so i can deny to the enemy. Also i rarely go warrior hero since in most cases it's a waste. I also play on very hard but i never had any problems with any campaign. I guess i know to well strengths and weaknesses of each race. :P