For all you heavy gunners don't use the guns that stick on to the bunkers you know, the auto-machine guns. Every time you use it a whole group of Nazi's come guns blazing toward you and unless your a sharpshooter your wasting your time and health. ):
When your on the last mission of "The Battle of the Bulge" go where there's a hill go down then you'll notice you can't go back up again. So send your men back then you'll see them walk through the hill.
If you're almost finished with a level and you havn't finished all the objectives, just keep walking around, and look in different places that you havn't been yet and you might unlock some new objectives. (hint: if you're up close to someone and you're reloading, press R3 insteading of waiting to reload)