I think that beast fort I mentioned might actually have been on easy – I just tried another beast fort, lost handily. Twenty-two cave bears wasn't enough to take out even the lizardmen's shaman. Or maybe it was just terrible luck this time. Huh.
I have had similar pathfinding issues to the ones Gnarz brought up. The AI also doesn't seem to recognize the danger of fire. I had my Keeper torch a library with a Flame Sphere to ward off a tri-faction invasion. It worked, but the moment I released control of the Keeper he ran back into the burning library to study and died.
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Expanding is worthless past a certain point. You can only have 40 troops total right now, so anything more than the tiles needed to sustain that is unnecessary.
You can obtain a massive amount of mana from killing things, and a smaller (though still significant) amount by torturing whatever prisoners surrender after a failed invasion. Try putting together a small team and raiding a village you've already conquered to kill the civilians for mana (you should be doing these raids anyway to get the final research tier unlock book for that faction).
Regarding pacing: the enemies don't seem to regenerate killed troops, so once you've survived a single invasion by that faction they are almost harmless (unless their faction leader survived). Mid-game is when you should be taking teams into the enemy towns to hunt down any survivors, raid shops and grab skill books. End-game is when you've killed every combatant in the elf, dwarven, lizardmen and one of the human factions. At that point you will be invaded by the Duke's incredibly powerful army (the other human faction).