I was loosing my AT guns crew, till I find how to use soldiers with AT rifles :]
If the range is not too big, skilled soldier with AT rifle is able to destroy gun and with this, German tanks are useless then- no other weapon, so the mission is easy then
Also, as it was mentioned few times ago, it's very good to take a look at the troops before a fight and assign the best shooters to guns etc.
After that- I start liking the defensive missions :]