There are about eleven balloons ahead of you, up and down and to the side slightly. Then, turn right and go until you cross a mountain, four to six balloons later. Then, turn right again. Blast balloons until time runs out. It is possible to get an "S" rank under the ace difficulty setting using this method, with just balloons, not counting the planes. When you start the mission, your radar will have blinking dots all around. The dots are actually the blimps and not the noise. This will help find the blimps' location and hopefully increase your score.