Mobile Artillary

I am having trouble with the first level for using mobile artillery. The goal of the level is to kill two groups of ogres. However, the big ones need multiple shots and they keep moving. How do you solve this?

attack ogers two times

is there a way to shoot two missiles at once. Or do I have to wait for the missile to hit and then fire the second?

try to attack behind stones or trees

Once you hit the ogre camp, the small ones die but the bigger one gets rattled and will come towards you. Predict its position and shoot. Thus, you will hit it twice and kill it.