So I've been playing for some time on insane/insane difficulty level and thought I'd share my experiences, and see if others also play at this level.
I lose most games so I've obviously got a masochistic streak . The AI's production and tech advantages are enormous, such that by the time you come into contact they will have a power rating 4 to 8 times your own - meaning they quickly declare war and you are soon warding off stacks of 7 man units with 200+ attack and HP with your just sovereign, a champion and some guys with spears and leather. Only the well-documented relative incompetence of the AI gives you a chance.
The trick is being able to survive long enough that you are able to catch up with the AI players from a tech/champion development point of view, so you can build a single death stack that is capable of defeating AI stacks without taking too many losses. Not easy to do, and a lot of it comes down to luck, but here's a few observations.
You need to get off to a flying start - meaning a good starting tile yield, good nearby resources and suitable locations nearby for early city spamming. I design my soveriegns with the Wealthy trait so that I can spam out pioneers to take advantage of city sites as soon as I find them. The advantages of city spamming have been covered well elsewhere but playing on this level means all of your focus in the early game is on getting to 5+ cities as soon as possible, everything else takes a back seat.
You need a ruthlessly efficient faction/sovereign build. I have won once using the Paragon exploit (yes that's cheating in my book), once using a dodge based sovereign (wraith, assasin, evade, etc to get dodge > 100) and once using a death build (convert every shard to death shards, cast dirge of ceresa to poison every unit for 50+ damage a round). I don't claim to be the master of designing these things but unless you've got a very specific strategy for dealing with the AI player's stacks of advanced units when you're only in the early-mid game, you're going to get smashed. Your trained units will be hopelessly outgunned so I think only strategies based around your soveriegn will really work - but I'd be happy to hear if others have successfully tried a different tack.
Cloud Walk becomes one of the most useful spells. You'll likely be only able to field one stack that has a chance of taking on the AI stacks. Cloud Walk lets you zip around and defend your borders from multiple incursions. Because the AI tends to send in no more than one or two stacks at a time rather than in concerted waves, you can bounce from border to border picking off the stacks as they wander in. Tireless March also helps with this, particualry if you've got a few champions in your stack and can cast Cloud Walk multiple times - you can teleport around the place almost at will.
Curgen's Maul is a very useful item. Being able to do Overpower damage when you are fighting a steady stream of 7 man units is a godsend. A decent combat champion will be able to take down a stack with a single blow - combine that with sweep and you have a mechanism for clearing out enemy stacks with comparative ease.
Sometimes winning can come down to sheer dumb luck. In one game against three AI players, the largest AI player fought the other two simultaneously. In a protracted affair it ground down the other two empires, by which time I had prgressed far enough to take it on. Nothing clever about that, but it does suggest that say starting in a corner and staying a bit small might encourage the other empires to beat each other up rather than focus on you.
Anyway, just my experiences to date. Anyone else play on insane?