From: Rio Grande do Sul
The DLC is really great. You have like 150 scenarios to play before you reach the last stand at Berlin (maybe? ;).
This gives you not only a lot of time to learn the game and the various units but also creates a little narrative story of it's own. When you look on one of your tanks and remember fondly how you drove it across France, through Greece and the Ukrainian steeps.
The original campaign is of course a lot shorter. You can check out the campaign tree by starting the game and go to the in game Library. Also the original campaign is a quite difficult. In the Grand DLC campaign the difficulty gradually increase.
I give you two hints that let me win on Field Marshal difficulty but fore more advanced tactics visit the Slitherine Panzer Corps forums.
In this game you can use a variation of the classic Pike & Bow tactic. With Pike being tanks and bow being artillery.
It works like that. You first stun an enemy, called here suppression, with artillery or a level/strategic bomber.
Then you attack him with your tanks. Because you stunned him he will not retaliate in full strength or maybe even not all. This will not only preserve the strength of your units, but also save you a lot money which you can then use to buy more units or overstrenght your units.
Early German tanks are very weak. Check the stats. This is really important. Learn the units stats (right click on a unit) and check the combat results (press L after combat) to understand how things work and why. If you hate the random battle results you can use the chessmode cheat which gives always equalized results. However this makes the game a bit easier. Bad dice rolls obviously hurts you more than it hurts the AI.
If you check early German tanks stats you will see they have a quite low hard defense value. Their ground defense is so low even artillery can hurt them. This will change from 1942 on with the new tanks. Eventually your tanks will become immune against most Russian artillery strikes. Because of that don't invest much infantry. I say more than 4 units is waste. In my last game I didn't not even bother anymore to buy any additional infantry. I just used the free infantry I got from the SS later as gift.
Because your early tanks are so weak have them backed up with artillery is very important to prevent the AI attacking them. Don't bother to use towed artillery instead just this Sig33. But beware it has a very low ammo reserve. If you're down to 1 round it's better to supply than ran out of ammo during AI turn leaving you're units without backup.
This combo tank+arty is so strong that often the AI won't even try to attack you because the fear to take heavy causalities.
About planes. Forget about Stukas. They do nice damage. No arguing about that. But later in the war, especially if you play the West campaign, the AI has so many planes your fighter will be busy to guard your units. You don't have time to babysit Stukas. Better use ME110 Fighter bombers. They are still so weak that they need escords, but when an enemy airwaves arrives you can use your fighter to kill the enemy fighter and your fighter-bomber to shoot down his bombers. This way you can clear an enemy wave in 1-2 rounds giving you often 2 rounds time to go wild before the next wave comes.
Strategic bomber on the other hand are quite nice. The later bombers are so good they can stand their ground against if ambushed by fighter Most importantly, like artillery they can stun enemy ground units. Plus overstrengh bombers are so powerful that they can completely destroy the ammo/full supplies of an enemy unit down to 1 ammo round + 5 fuel.
You can use this in 1944 and later to delay a powerful enemy tank moving up, or in a defense scenario if you corner it with two units, leave it stranded and use it as barricade. One hex less the enemy can attack your line. ;p
One last trick. Each DLC has a experience cap so you don't get supper 5 stars unit immediately. When your units hit the experiance cap consider not to deploy then again, but instead buy a new unit. It's easier to train a fresh unit in '40 than '44 again mean Russian IS-2 tanks. For example I finish building my core fore in 1940. Except for a few Anti-tank units I buy later since the PzI anti-tank is completely useless unit.
My final core force was like ~ 3 Inf, 3 SS-Inf, 15 Tanks (10 Panther Tanks 2 Tiger I, 2 Tiger II. I lost one later :( ) +6 SS tanks. 4 AT Tanks, 16 Artillery units, 2 8.8 AA guns (you get later a lot AA units for free in the east and west campaign), 9 fighters ( was 10 lost one :( ), 5 fighter-bombers (was 6) and 4 strategic bombers (here I lost one too).
Ups, look at this wall of text. I got carried away. Sorry won't happen again.