Oh that's a hard. Was playing it when I was 11! it is the mission where the enemy uses for the first time long range artillery. That is why it is so hard.
Go first just a little bit to the north and there steal some mortars from the enemy.
Also you can assign your snipers wit "W" and right click on an enemy bunker to shot at enemy personnel inside. Even if the bunker has never prior to that entered your line of sight. I didn't know that when I was playing.
The rest is unfortunately not much from me. First, try to capture the railroad station and those German weapons and vehicles on the station and as much as you can of long range artillery.
Then when you have it go to north, put your sniper on cease fire, (snipers have a good line of sight) and from distance bombard everything the snipers manages to see. Also bombard all buildings preemptively, this game is no place for giving remorse.