At the beginning of the second level in the Water Benders Campaign, build a large army consisting out of 3 Kyoshi Warriors, 6 Mounted Guards, 2 Earth Benders, 1 Master Earth Bender and 2 Guards. When done you can attack the Fire Nation settlement with ease. Remember the settlement is in the top right corner of the map. use Momo, your air patrol unit to scout the area, but do not get to close to the towers without any backup forces or your efforts will be in vain.