If you’re able to beat the AI at harder settings, please feel free to post your strategies and such. I am not only happy to incorporate your strategies into the AI but relish doing so.
Okay, first at all - i spot 2 different switchers of difficult, in 1 and 4 screens of starting new game.

I start on extreme with 2 factions near me, my first city have good placing with 1 food, 1 gold and 1 lost library sources. Then i have build start buildings at lvl 1, some scouting by sovereign and Janusk and spot some good place for second town. Early even than Peasant i trained my Pioneer and heading there.
But when i come - place is being occupied by other faction. Dam, huh, i am later! But i spot, that he place town in middle of resources, and it be first lvl. So, he cannot reach em for some turns. Then i move my pioneer forward, and place town directly near resource i need (food), but between enemy town, and resource (actually, i some stupid around for range 5 cells from other town). I take non-aggr treaty with them 2, bribe em some gold, because their cities stand on my path, and i cannot go through without war declaration and cannot spread my kingdom.
Then i train some peasants, and one more pioneer as early, as i can. Spells research too very easy, and i take my first summoned unit - familiar too early, at my taste. So, i scout around and spot other 2 factions (4 atm / 8 at all). I put all my research points into Civilization and first at all learn housing, then gold productivity and then knowledge.
Phase 2, i think
So i very fast take out world knowledge about housing,gold and gathering knowledge, build 2 more cities, bribe pair heroes, without any idea as i can use em, just for count, because i have about 500 gp, and don't know where spent it. Hero have some army, with combat rating about 150 (only summoned creatures and clay gollem from whistle). I still not in war an any foes.
It interesting moment, because some past of it - two factions one by one declare war to me. So i be very cautions, and anxious about no defense in my towns at all. I hope start training peasants before enemy comes to conquer me, but i remember teleport spell, and it work well, i can deliver my best squad to city under attack. So i switch off training tons of peasants.
I haven't look any enemies come to me, i don't know why. Instead i capture two weak towns from one of em (and he want declare peace) , and i spot message, that my other enemy just game over by someone. I even didn't seem em at all, maybe i am just miss it.
Soo... phase 3
Without wars at all, with 5-6 cities and tons of gold and knowledge i learn some first step in diplomacy (for taking benefit from tech and trade alliances), and put all my knowledge to Warfare. First at all i finish thread with weapon, and can build units with Lord Hammer. Ololo it 40 damage, i like it! And i discover last armor in game, still without wars.
After then one faction (my girl next door
- first faction which i meet in game) - declare war on me. It strange, because we have some kind of neutral relations and non-aggr pact (maybe it just end, because 99 turns passed). I again start think - where i can take some troops, for defending. Because all his towns be near, and his heroes and army scouting very close.
Atm i have about 1-3 heroes in every of my 5-6 town, and about 10k gold (because i don't spent it at all), so i dress up my sovereign into heavy full plate and take Lord hammer. Bingo!, 1 hero have combat rating about 200-300, from zero. Huh, i just dress up my 3 heroes in border town. They be merchant, wiseman and someone else (not warrior or assassin), all lvl 1, and attack by em (without hero) army of my enemy. It about 1k combat rating against over 200-300 (big army, with pair of parties, and pair of heroes). I wiped em without any damage.
So, i get pair of towns (okay, maybe five), and make peace with em, i'am peaceful ruler 

After this - no one not be able declare war to me, and for 36 turns, 300 materials and some gold i train 12 avalanches (melee troops with these hammers and plate), so 1 troop have combat rating about 1500, and really devastate dragons with bliz wins. So relations with factions slowly raise up 
After all i end my knowledge tree, and take 7 or 9 lvl in spell book, i don't remember. Think about conquer world, by army of 3x12 avalanches, but it have some glitch about disappearing my men, they just dieing at march, i don't know why, even without casualties in battles.
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So, some resuming here:
1. As game advice - first at all summon all what you can and lead it by sovereign. And be prepared teleport it to city under siege
2. Take non-agression pact with all foes (even if you conqueror - chose time to attack by yourself)
3. Learn 1 food, all housing and all gold production (gold can buying anything)
4. Bribe all champions, too much as you can, for further (they bring profit too)
5. Learn equipment and be able dress-up even weaver for battle (you can take 300 combat rating per champion just instantly, w/o waiting 36 turns)
6. Take towns of you enemies without defense and siege, and another part -
7. don't attack troops of you enemies (c)Sun Tzu - "Art of war", i think
Sorry for huge posts, but it hard say about whole game on large map with pair of words. And sorry for my english too of course. Cya 