You should have checked
www.ratemyprofessor.com before signing up for that class. But honestly like others have said, you should know the concepts and think of questions that he might ask on the exam. We've all had difficult professors but all got through it.
If he passes back the exam then you should compare it with those that got the A's and B's. Also, if the professor stresses a point make a note so you know it's important. Another thing is that you guys might be subjected to groupthink. Perhaps you should study with people that actually know the material and are not afraid to "think outside the box." Sometimes you would have people in a group that just go along with what everyone else says. If that's the case then what you know (or think you know) is limited to what was discussed. When I study I would always stop and think when I see a new concept or something that I might have been confused about. Tying it all together helps tremendously.
Going back to your original post, I noticed you said "the prof is a nice guy but everyone is doing bad in his class." Could it be that those that got the bad grades thought he would be easy since he's nice?