When you get then mission to knock out a large gun across the river from the Eiffel Tower, nearby on the same building is a smaller anti-aircraft gun. Eliminate all the guards nearby but leave the AA gun intact. When the mission is completed you can use the AA gun to shoot down Zeppelins. This will give you a level 5 alarm, which causes a series of airplanes to begin strafing you. Zeppelins earn $50 and planes $100 when shot down. The targets are unlimited, allowing you to get as much money as desired. To finish, jump off the building and die. When you return to HQ, you will have lost your weapons but have plenty of money to replace them. This can be repeated as many times as desired as long as the last AA gun remains.