[Imp]

As others have said stay away from the best gear or choose the underdog, you can set up the battle & run it 3 or 4 times with the AI set to buy your stuff to see what should be a reasonable mix of units.

If you want a few nice toys & its a largish battle set them up in reserve near the map edge as quasi reinforcements, cant move till turn 10 or some such then have them come to the rescue.

On the first turn or two do nothing in meeting engagements, let the AI go first.