In the original game it's pretty easy, but I don't know if FX makes it harder. In vanilla you just wall yourself in as soon as the game starts: extend the north wall of the pre-made dungeon west and east to meet the rock prominences. Stay there training and researching until the money is running low, then break out at the eastern end. Do not waste money on traps because it's a war of movement, and IIRC you can't build a prison until you capture one. There's one in the east if you start that way. Once you can build prisons the strategy is to grab every enemy you can and imprison them, aiming for skeletons. You don't have time to convert them, so don't bother with torture chambers. You want an army of 20+ skeletons. Then it's just a slog to clear the middle area. You need to do it quickly because money is an issue for the whole game. Have imps tagging every bit of land in sight, but make sure you are only fighting in one place: it's a big area and you can finish one fight before moving on to the next.
Finally there's a fortress in the north. It has a LOT of traps. But it also has a gem mine. It'll take a while to clear, but its' not hard.
M