When using AT guns of ANY experience level, I often "stick" them to an ambush marker directly BEHIND themselves so that they cannot fire unless directed to do so (under almost all circumstances, there are of course exceptions). This way, you might happen to miss an opportunity for a flank shot on that one nasty heavy tank that you really need to kill, but you will never fire a "weak" (17% hit chance, no kill) shot, exposing the highly vulnerable, ever precious guns. If they were placed properly to begin with (yes, the terrain can be impossible sometimes), you should eventually leave your opponent no choice but to expose his flank. Guard the guns with infantry, at least 40 meters in front of the gun, set to ambush. This way, no infantry can sneak up on that “locked” AT gun and kill it without triggering the infantry ambush. Keep that AT gun hidden! To kill that infantry as an attacker, your opponent needs superior force to win. If the infantry are well placed, that very well may mean the support of armor or other valuable support vehicles (this does not account for the wrath of god---artillery). So, set up a grand style ambush. Place the AT guns and infantry so that when support is brought in, the AT guns will have that killer flank shot from the other side of the board, if not a nice shot from the gun you are protecting. Check all LOS possibilities. I'm always surprised how quickly people setup with these IP games. You can really spend a hell of a long time checking all the angles and really designing a solid crossfire!
And Chad is right about that stealth bonus. Try his favorite as the Axis and get some Fusilier SMGs (for that killer 40 meter punch) and a great stealth bonus for an ambush in the woods . . . but at the BACK of the woods (so the opponent thinks it is all clear by the time his troops reach it). Surprise! You're dead. Nothing can touch these guys at 40 meters.