I just beat insane difficulty twice in a row with different character builds and thought I should post a bit about them so they can get brought in line with the other builds.
I custom made both characters of course and their factions. One was a death knight and one was just a knight with no spells.
Race: Tarth is the most powerful with +3 init and attack. You need the extra strength at low levels.
Faction: I always take all 4 weaknesses and take Quick, Lucky, Master Scouts, and 2 others. For my deathknight I took death worship for the amazing Corruption spell and for my non-caster I took heroic and scholars. Civics would be good here also.
Of the faction options Master Scouts I always take on all characters, it is the strongest choice by far and does 2 great things.
Profession: I took armorer for both. I think that general would be ok too, but not quite as good. Warlock is ok for some builds but casting damage spells is a bad strategy at impossible difficulty because everything has triple HPs.
Talents: Both took Hardy, Discipline, Wealthy, Brilliant and the crown equipment.
My deathknight spent his last 2 points on rank 1 death magic. My non-caster took tactician and might.
I took clumsy as my weakness. It is easy to work around and having it makes combat more fun for me.
Of these only Wealthy is really overpowered. (start with 1000 gold). It should be 400 instead.
The crown equipment is great if you have lots of spells but it was terrible for these 2 guys and I might skip it next time.
Any spell casting line except death should take the crown though.
Map Settings:
Small map, 3 opponents, insane difficulty, dense, fast, dense, dense.
I didn't use ctrl-N to reload starting location but I would have if I had zero essence or something really bad. I had 1 essence in one game and 2 in the other. Essence makes your researching of leather armor faster since you cast inspiration on the first turn.
Strategy:
You need to split your 2 starting heroes and level them both up (exp between heroes is split if they group up). To succeed on this you need to buy them full leather armor, then shields, then horses or wolves.
Get leather armor first, then get civics to rush buildings, then get horses.
Rush out some settlers and some cheap buildings. Save enough money to buy 2 horses of course. I save a bit more than that.
Kill computers if they got a bad start and you can wipe them out. Otherwise don't make war with them.
Late Game:
The richest computer will research super duper magic platemail. You need to save your money to buy a set or two of this. I was able to visit him and buy the best pieces. You can't research it yourself because you took all 4 weaknesses when designing your faction.
To shop from him just walk into his lands while not at war with him.
The computer will have infinite troops, so try not to fight him. I won both times with quest victory which is fairly fast and fun and it means you can basically ignore the computer.
Both times I built zero units and recruiting only the hero that starts in sight of your town.
In one game I killed 2 of my 3 opponents and tried to kill the third but he had infinite stacks of infinite guys and he killed all of my undefended towns. When I won with my non-caster I just had 2 heroes and zero other stuff and got quest victory. I was level 25 and had the best sword in the game. It gave me the bear special attack where if I hit I get to swing again until I miss. But I lose 6 accuracy per swing or something.
When i won with my deathknight I had lots of towns and 9 death nodes because I was using corruption to turn the nodes all to death.
Right before the final quest fight I sacrificed every town so they had zero population and I spent 500 mana destroying the boss he can't walk to the door it turns out. I could have beaten him other ways though with my 48 point drain life and great debuffs.
Thats it, I think damage casters and summoners are a bit weak and would not have won at this difficulty. Units could have won eventually I think they get strong if the game goes long.
Hope it helps balance or other players considering harder difficulties.
Mike D.