Going Rambo?
 

I always enjoy getting a small group together (5-6 men) and pecking away at an enemy's defenses. You know, sneaking up in crawl mode and having a sniper kill one or two people. However, many of the missions don't allow this. What is the best campaign for this? For example, in the German campaign, the enemies keep bombarding my troops with artillery, even though they are lying down in grass! How do they know where they are? I just want those guys to stay hidden somewhere while my small group does what I want it to do. Is there a way to make them retreat off map or maybe somehow keep them from being spotted by artillery?

Single mission 'Behind enemy lines' is best suited for this kind of gameplay.
Which German mission you describe?

The first one.

It all depends on mission design. In this mission mortar fire is called upon general area of German forces.