In Buckholz (the second to last mission) when the mortar crew and flak guns attack you after the "Defend your squad" objective... Instead of following the soldier as you are instructed, run to the right-hand side and take out the flak gun crew. Then keep going around the corner to the right and take out the mortar guys in the building on the corner. But DON'T follow the soldier as he will lead you right through a bunch of enemies with machine guns.

Also, don't loiter in one place or the big guns will take you out with no warning, then you'll be forced to fight that loooong battle again ("Defend your squad") before you can attempt this again.