AT cannons are the scourge of tanks. Their penetration quotient is high, especially in the case of the heaviest, AA versions. These are perfect for destroying enemy vehicles. They have high penetration quotient, deal considerable damage to tanks and are highly accurate. You can easily take them over by attacking their flanks or the back. Infantry has the tendency to attack crew of cannons, so if it is not turned in your direction, where the crew are hiding behind armoring, you stand a chance of quick elimination. Sandbags provide additional resistance to the cannon, or absorb some of the damage. High concentration of cannons allows you to eliminate opponents quickly before any tank responds with fire. Try to position cannons in vegetation or behind buildings, for appearing tanks to be an easy target to them.
Always try to maintain additional infantry units near your cannons, thanks to which they will target enemies faster. If you have not deployed sandbags, cannons will automatically rotate towards the enemy. If a light cannon has a complete crew, you can push it into a different position. Any cannon can be attached to a truck to haul it into a different position (or to lightweight vehicles, if you have the towing hooks skill).
Tank destroyers are self-propelled, threaded AT cannons. Usually, they have thick armors, on par with that on medium tanks, as well as a strong AT cannon of caliber better than those in lightweight or medium tanks. Their speed is sufficient and they can quickly retreat to a better position. It is best to position them among buildings, or covered with forests or ruins. Their greatest downside is lack of rotating turret (some lightweight models have one). Such a vehicle has to move to target the enemy, as a result of which, it can expose the vulnerable sides or back. They are not too effective while firing in movement, but are excellent for defense, assault in the second line or, while flanking armored forces.