When you get the mission to destroy the large gun across the river from the 
Eiffel Tower, nearby on the same building is a smaller anti-aircraft gun. Kill 
all the guards nearby but leave the AA gun intact. Once 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. Once you are done, jump off the 
building and die. When you return to HQ, you will have lost your weapons but 
have lots of money to replace them. You can repeat this as many times as desired 
as long as the last AA gun remains intact.