Yes, yes, listen to Flashburn. I had trouble with this campaign too when playing as Germans. I almost literally got steamrolled by those KVs and T-34s when they appeared. The key to success is to ambush them and engage the armoured and infantry forces with anything but your tank destroyers. Keep them in the hiding and roll them out only after you're sure some of enemy tanks are knocked out our routing and the rest of them is very busy with something else (shooting your troops).
Here's a short image-based AAR from this battle I once posted. It shows the location of my troops before the clash. My casualties were great, but enemy advance was stopped. (
If you don't want spoilers, don't scroll beyond the scores screen!!!!!)
http://img850.imageshack.us/img850/1092/shortaar.jpg