Hold the Lighthouse and Barracks, and out kill the other team. This is very doable, but it requires good teamwork. Keep your best team at the objective near their spawn, and the rest at the objective near your own. Hold these two points and crush any stragglers who venture near you.
If at any point they get desperate and send their entire army at one of your objectives, that is when your team from the other objective should swoop in and grab the Fort. Many teams will so desperately cling to the "HOLD THE FORT AND WE WIN!" strategy that they will never send more than 3-5 people to try and take back Lighthouse or Barracks. If this is the case, you can win a very satisfying victory by out killing them without ever holding the Fort. This is my personal favorite way to win, as it shows people that intelligence and good strategy are rewarded in WAR over simply following one method by rote.