I had sent demon a PM, but I can reprint it here.
After doing some research, I found out that the rules that my family has always played under are actually a variation of the official rules. In our game, when there is a tie, the piece which MOVED wins. (In other word, the tie goes to the aggressor.) In the official rules, however, both pieces are lost. Hmmm and double hmmm. I also seem to remember that in our version, a Spy could kill a 1, 2, or 3, but lost to a 4 and below...but I think the official rules claim that he can only take out a 1. Grrrr.
Well, here was my unbeatable strategy for our variation, but you can see how this might help you under the official rules:
1) Place your flag in the middle of the board mostly surrounded by bombs. There should be two "doorways" on either side.
2) Immediately adjacent to one doorway, place your 1 and a 4. Immediately adjacent to the other doorway, place your Spy and a 4.
3) Put a couple of key pieces inside the fortress as well, including your 2 and a couple of 8s.
The basic idea is to play a waiting game. Challenge anything coming across with a weak piece you don't mind losing so you know what's coming. The goal is to a) keep your bombs intact around your flag and b) bait the enemy into sending a high-value piece through one of the doorways. (You can even make it obvious about where the entrances are by moving pieces out from your fortress from time to time.)
The trick with this layout is that any enemy piece must step through a doorway on one turn...but on the next, you have a free shot to take the initiative. In our rules, this means that you always will win the battle! (Your 1 can take out anything except for a Spy...but if you know it's a Spy, the 4 is ready. On the other door, the Spy can take out anything above a 4, and the 4 would beat anything else by moving first.) Once you've bled off enough high-value pieces, you can take your time walking through his forces -- you'll have the highest-ranking people on the board, so as long as you don't hit a bomb, you're in business.
With the official rules, this becomes a little more tricky. You might be able to apply something similar...but I'd have to think about it.
Anyway, sorry to get your hopes up. I'm going to lecture my family at Christmas over teaching me something OTHER than the official rules.