From StrategyWiki, the free strategy guide and walkthrough wiki

Primary Objectives:

• Destroy the East and West anti-aircraft guns
• Escape the city
• Destroy the rooftop anti-aircraft guns
• Find a radio and contact OSS Control (Campaign only)

Secondary Objectives:

• Obtain Axis troop deployment plans
• Disable Axis officer car in the North District
• Disable Axis officer car in the South District

You'll have to search at least two German cars that can be destroy if you can find it through the level.

The Artillery could be hard at first but you can still get the hang of it. To move it left or right, move your in a circle either clockwise or anti-clockwise and swing up to fire. On the Wii Zapper, you move the in a circle and press to fire.