I wanted to post the build I used to beat insane. If you want to discover what works on your own you can skip this post.
There are probably other strategies to do it also.
Insane is really really crazy. The weakest groups of monsters have 150 hp. You have to be able to beat those at level 1 to work your way up.
On insane the computer was completing world achievement buildings around turn 10. Before I even had a second city.
When I encountered the first opponent he had all 3 of the spell of mastery buildings already built. (around turn 20)
I tried many strategies before i ended up with this one.
The basic idea is that I would craft archers with as much DPS as possible and have a hero tank while they kill everything.
Quick: +10% init
Lucky: +25% acc and dodge
Archers: Unlock special bows
Enchanters: Scrying Pools add 1 essence to your city
Tarth Blood (gives you 2 shots at -30% accuracy) +3 att/init if in a stack of 3 or less.
No spellcasting, hire guys or purchase books of spellcasting for 90 gold
Clumsy: can hit adjacent allies in melee sometimes
Discipline: 1 acc and spell resist per level
Wealthy: 500 gold. Remember the computer has all techs unlocked super early and you can buy everything from him
Hardy: 1 hp per level
Tactitian: 1 init for army
Scholar: 10% research
Might: 3 attack
Rusty sword, procipine's crown
Your first town should have 2 essence, add another with a scrying pool fairly early on.
This gives you 3 mana and 6 gildar per turn. Make this city a fort, you need to make archers asap.
Learn Fire 1 by purchasing a Tome from Queen Procipinee if she is in your game. You can use her shop, they cost 90g.
If not then hire a hero with 1 fire and a hero with 2 air as soon as you can.
Enchant your city with +1 fire damage per essence and +1 initiative per essence.(get rid of your mana enchant)
If you can find a wolf pit get it producing wolves fairly quickly.
First learn the tech for +1 research buildings.
Then research wolves if you found a wolf pit, then research bows.
Research +1 movement speed spell
Resarch roads between towns
Research +1 army size
Research better bows
Research +3 frost damage amulet +3 fire damage amulet.
Ideally your archers have decent bows, wolves, +3 frost and +3 fire damage from amulets.
Ithuane Longbow, Warg. Amulet of Frost, Amulet of Flames
Tarth blood (can shoot twice at -30) 5 turn cooldown
Fury -1 hp, +1 damage
Bloodthirsty (+25% damage vs injured enemies)
Underdog (+3 dmg vs higher level foes) the final boss is level 20
+3 damage from city enchant
+3 init from city enchant
This means a unit of 5 archers will display 100 damage (20*5) and do 115 vs higher level, 144 vs injured higher level.
They shoot twice on their first action so up to 288 damage.
Even just a few of these archers will let you beat almost anything.
They have 25 init, but if your fortress town hits level 3 you can give them 'go first in combat' as an option when the town levels up.
Don't piss off the computer opponents. Accept any offers they make that you can reasonably afford. Don't build the spell of mastery buildings, they make the computer angry and they can wipe you out.
Try for quest victory. It is the easiest at this difficulty. (its still hard)
The final boss (Waerloga the dragonlord) will cast an AOE life steal that does 60 damage to each of your stacks and heals him for that amount.
You need to counterspell and the dragon has 85 spell resist so your counterspell hero has to be amazing. Try to hire a hero and groom him for this task. Giving him a book to learn fire magic and ranking it to 3 will give him a counterspell. Items like Emerald skullcap give 5 spell mastery, you need to have well over 100 total.
The other option is to give your archers a lot of health so 60 damage doesn't flatten your army. Change items and talents around to do that.
The dragonlord at the end of the quest has 140 spell mastery so he is hard to resist, better to counterspell or stack health.
Only keep 2 heroes in your stack, the rest should be archers. One hero is your tank and the other is your counterspeller. My counterspeller was a commander that added to the teams accuracy and experience points.
You can buy really good champion armor for your hero from the computer towns. It doesn't matter that much though the archers kill everything before he matters much.
You could probably do almost the same strategy with wands instead of bows. I didn't do the math on it but wands ignore armor so they are probably similar overall.
I won pretty quick, I had 2 level 12 heroes at the end and a few level 5 or less in towns. I only had 5 cities,
I got the highest final score which is 10000 or something. There are no achievements for insane,
I played insane opponents and insane map/monsters. I used 4 opponents and a medium map. I had above average on almost all settings, more monsters, more events, slightly faster research and building, more quests. I have more fun with those settings, less waiting around.