Challenge mission isn't all that challenging...
...for the Germans that is. Once you get two 88s up, arty, mortars, more bunkers, sniper, sandbags, maybe an MG crew (optional), and of course AT guns.
This is an awesome defense that the Soviets cannot break. They didn't even get close, only my bunkers received some damage and they were repaired on the spot. The sniper acts as the visual on the battlefield for the 88s so they can do what they do best and cripple enemies. It's like easy mode.
The Soviets on the other hand, I barely held the base and just hoped that timer would run out quick. I won, but just barely. Soviets just aren't the defensive type, so even on easy mode I couldn't hold the Germans back.
I am possibly not doing it right. Maybe someone with experience playing Soviets can help me out.
I used commander, vet mortars, vet ATs, firebase, sandbags, some mines, trenches, and tanks (SU-85 upgraded). I was thinking about tank hunters, but I decided not to use them.