Also, pay attention to how morale works! During your turn, always fight the small, easily-won battles first! Every little victory (esp. getting a town) gives a little, and weakens the enemy's morale (the one who lost the battle). Morale can turn the tides.
Victory depends on: location you've chosen, and early gameplay. First annex nearby towns, then attack nearest neighbour from two directions if possible: first should conquer enemy towns, second should flank the capital (use one as the diversion for the other when you can). Also, it helps if you can get a farther neighbour to sign a pact. It's fairly important to conquer your nearest neighbour, so it won't cause you trouble later on, and you can focus on the (hopefully) one-vs-one war from here. Note that the enemy capital (late game) may seem to be guarded heavily, but it's pretty easy to make them jump out of the bushes, if you attack with enough stacks. Diversions work well.
I've only tried the game on normal so far, though.
It may only be the case that I am simply not skillful in this game, but I hate it when the AI abuses common human mistakes and flaws (like getting confused when he fires the switch-places gun) which doen't bother a well-written AI. The game is really frustrating when the AI beats you the same way every time.
Also, f you. For no reason. Just for the sake of it. F you.
Rant: This reminds me of Liquid War. And I hate level 13. F you for that level.
Constructive criticism: it's too easy to botch the unit assignment. I order a unit to go somewhere far away, then try order another far back. What happens is that the unit far away goes back. This game needs better controls and distinguishable units, select-wise. Also, disable the auto-engaging with ordered units, keep it on with idle units.
It's either poorly implemented (after you push space or down button, you get a small delay before actually doing the action - same thing for releasing button), and that's why it's hard, either on purpose or accidentally.
4/5 because the developer is a wiseguy.
If you die apparently for no reason, having a lot of hp, it's because the zombies destroyed your base. Above (or under) your hp is your base's defence, and it works like your hp.
Copy this comment to keep it alive, cause a lot of people don't understand.
You can get power-ups by making synergy pairs, putting 10 same kind of symbol/color in the circle, and do other "harder" things :). This is what I've experienced.
 "Hex Empire"){kind=icon}
 "Chaos Faction 2"){kind=icon}
 "Pixel Legions"){kind=icon}
 "Mr Runner"){kind=icon}
 "The Endless Zombie Rampage"){kind=icon}
 "Ring Pass Not"){kind=icon}