Current "Can't Lose" Strategies
Elemental Forums
I'm starting this thread to document how we, the players, are currently winning. Then Stardock can take note and adapt to make the game more challenging for everyone. 
So far all of my victories are on "ridiculous" AI settings. I favor smaller maps, but I've won on large with the maximum number of opponents as well.
Diplomatic Victory:
This is actually pretty darn easy. I would rate it as easier than conquest victory because you don't have to bother with hunting down all the AI Sovereigns. Play as Kingdom. Make sure "Treaties" in the Diplomacy tree is the first or at least one of the first technologies you research. Also make sure you build one observer ASAP in each city you found. When you first meet an AI, on that very turn if possible, sign a treaty with them. Non-aggression, tech, money doesn't matter (personally I do all of them if I can). The trick here is getting the treaty signed early so they don't have time to form an opinion about you. Having just one observer in each city will, shockingly, put you in the "your military is stronger than mine" category for quite awhile, so getting the treaty signed shouldn't cost you more than 10-30 gold. As long as you have that one treaty going you will never slip below "warm" as long as you keep your military relatively equal - and again, you can do this by just cranking out cheap units with an attack rating - no one is ever going to attack you so actual combat effectiveness doesn't matter. As soon as you start having children grow up marry them off. Your goal is to get at least one child married to every other faction. Once that is accomplished you are set, the only question is how long it takes you to raise the obscene amount of gold each faction will want to sign the alliance treaty. The game does raise the "base cost" of treaties based on how many you already have active, so expect to pay out the nose for the last few.
Conquest:
Many other people have already posted on the relative ease of steamrolling the AI with champions/summons/archers whatever. One thing I will point out for fellow war-mongers is stop bothering with rushing to research archery/advanced archery - just use the faction creator to make a custom faction with the "Master Archer" trait. It only costs 2 of the 10 points you get when creating a faction, quite the bargain. In fact unless there are unlisted benefits for the pre-built factions making your own is, at present, far more powerful than using the in-game ones.
I've stopped using archers for now until the AI can put up more of a fight.
Spell of Making:
This one is just tedious, I've only bothered with this once. As a side note, are there any spells in the game after level 10? I researched level 11 but saw no new spells, so didn't bother with 12. Also I found it silly that on a large map, with all factions as opponents, the world contained exactly FIVE shards: 2 fire and 1 of the others. I mean, in order to capture and control them I could have just won through conquest anyway. That was in 1.08.
Master Quest:
This was actually a bit of a challenge as far as some of the battles. You'll have to run all over the map (which is no problem if you have non-ag packs with everyone) but the real problem was the final fight that includes a guy with 100 defense. Liberal use of the Deflect and Berserk spells got me through that.
I will update after another couple hundred hours of gameplay 
In the meantime, how do you win?
point of view.
. Medium map, four AI. The first three were on my west, south and east borders right from the get go. Seeing how treaties was the first tech I researched, I did as you suggested. I also put observers in all of my cities. The moment the treaties had expired, all three AI declared war on me. I was forced to wipe two of them out, and was working on the third when the game crashed big time. At which point I decided to reboot and try a new game. I'll try for a dipolmatic victory next game. I've just won my second game (master quest) which was a lot of fun.