I have beaten this one on hard; definitely tough. Here's how I beat it:
Setup the four AT guns like you said. Call in 2 tanks and then special squads. Send the special squads up the hill on the far right but keep them on your side of the bridge in prone position. They will stop the enemy flanking maneuver.
Your tanks and AT guns should handle tank assaults. The problem is when the enemy brings in artillery. It's a little dicey, but send one of your smaller tanks across the center of the river (otherwise the ice will break) and try to get a shot off on the artillery. The artillery is weak, so usually only takes one shot to kill. This step is essential to your survival, so keep saving/reloading until you manage to sneak your tank in just far enough to get the kill shot. Definitely time it so you send in your tank between waves.
Once you kill the artillery it's pretty smooth sailing. They will continue to attack with tank/infantry waves but your line should be able to handle it. Try to reinforce continually with tanks and special infantry squads.
I ran almost the entire mission at the slowest speed. I would love to watch someone who could beat these missions on hard playing at normal/fast speed!
Post back if you can't get this strategy to work. Good luck!