[Tomalak]

Well as previous posters noticed, luck plays a big part here, since a loss of few AT guns can mean a difference between victory and defeat. Save often, so you can go to earlier save if, for example, a lucky shot takes out one of your guns.

Take out tanks first, as soon as they are in range, take MC of one of your guns, and start firing. If needed, lower the game speed, so you can manage everything you need. Might be a good choice to reposition those two guns you get in the beginning, which are standing in the middle of the road. If you have problems disabling the tanks, try to take out their tracks at maximum ranges, they will still pose a threat, but at least they wont be able to charge at your line. Keep in mind, however, that the closer a tank is, its easier to destroy it, so try to find good balance.

Call for aircraft attacks, and new infantry whenever you can. If you position infantry reserves behind the trench, in the bushes/cover they can fire at the enemy, and be ready to reinforce where needed, without being in the first line of fire.

The main danger comes from enemy aircraft, since they are capable of destroying entire squads of your infantry, so pay attention to the mini map so you can see if a section of the trench becomes empty. Also, always keep someone on those stationary MGs.

For me, the first part (defense at the first line) was the most difficult. You have that big coastal gun during second part, plus you can get a AA gun, which you can conveniently put near the bridge, where it has a good field of fire, and will take care of airplanes as well. Also during the second part, its possible (with some luck) to repair a German tank or two, which can serve you well before your own tank reinforcement arrive.

So, in short:
1. Save often
2. MC your AT guns as soon as tanks are in range. If you have problems destroying them in short time, target the tracks (you can disable a few tanks then hit them with air strike, especially since infantry likes to gather around).
3. Use reinforcement points on infantry (rifle and smg) and air support. Get the AA gun in 2nd part.
4. Keep infantry reserves behind the trench, and watch the mini map to see when reinforcing the line is needed. Keep those stationary MGs maned.
5. Use air support often to target concentrations of troops, tanks especially.

Reload if things are going badly, like if you lose all your ATs in a single wave, or similar. If you're in a position that 6 enemy tanks have reached the trench, or, god forbid, managed to go even further, like keeping the reinforcement bridge under fire - you've probably already lost.