Having a few problems...

[Sneaksie]

Dunkirk is tough. First of all, take your time.
Choose Pz IV tanks preferably, because they have HEAT shells capable of penetrating heavy french tanks.
First, stay were you are and defeat enemy counterattracking armor. Then advance your armor slowly on the left flank, while sending infantry to take out artillery battery at the center. When you spot the location of enemy gun on the other bank, concentrate fire of your tanks on it (use Ground Attack if you don't see the gun itself). Hopefully, your infantry will kill artillery crews on your bank of the river and there will be some soldiers left - order them to now empty guns and use these guns against enemy too.
You should receive reinforcements after you suffer certain level of casualties. When French tanks arrive, use Pz IV tanks and captured artillery against them.

You can also choose some artillery guns before the mission and use them for supporting the advance (pushing it forward for a short distances and firing at appearing targets or Ground Attack enemy gun positions).

Good luck!