Okay, I have now finished the first seven scens. At the middle diff because I'm a wuss.
1. Lake Khasan 1938 (border conflict, with Japan, Manchuria)
Good first scenario. Used two about equal prongs. Conscripts are really fragile, but they are also very cheap, so if you use them accordingly, they are ok.
2. Khalkin Gol, main Japanese attack in June 1939 (border conflict with Japan, Mongolia)
Imo the most difficult of these seven, probably because I played somewhat like Bru first i.e. too aggressively. The easiest way to win is to use enough support for the "orange guys" (esp. for the northern group) and only take the northern and southern bridge near the end.
3. Khalkin Gol, Soviet counter-strike in early August 1939 to defeat the Japanese
With your overwhelming armoured force it is quite easy to strike from NW and SE towards Nomanhan where the prongs meet completing the encirclement.
4. Grodno, September 1939 (vs. Poland)
An easy respite which only requires you to keep up your speed because of supply issues. Many southern supply towns are unoccupied.
5. Invasion of Karelia, start of the Winter War vs. Finland, late 1939
The winter conditions really slow your forces (except armor) so stay on roads when possible. I was somewhat disappointed to the counterattack by Finnish ski troops. I think they should be more concentrated. As of now they are too easy to destroy piecemeal.
6. Summa 1939, Finnish attacks after the 1st Soviet attempt to defeat this key part of the Mannerheim Line failed
Nice and rather easy defensive battle. The Finns have only one armoured unit so you don't need AT. Retreat all aux units you can around the primary objectives and defend them with your core infantry and artillery. Tanks can make destructive pinpoint strikes against the advancing Finns. BT-7s excel in this because of their speed.
7. Summa 1940, player leads the second major attempt to break through at Summa
A frontal attack against a fortified line. Choose where you want to break the bunker line (my recommendation is the eastern end) and roll the enemy line. Fast tanks are very useful in taking the two secondaries because the terrain combined with winter weather makes other units probably too slow. I used the troops taking SW secondary for the primary of Kuolemajärvi. This is also a safe area to use your bombers (if you have any), because there are no enemy fighters here.
My core at the end of Summa 1940:
Infantry: 2 x regulars, one each of heavy, marines, paratroops, engineers
Armour: 3 x BT-7, 2 x OT-130 (flame)
Arty: 3 x 122mm A-19, 1 x 122mm M1938
Air: 3 x I-16, R-10 tac bomber, DB-3 str bomber, 2 x R-5 recon
Not deployed: 45mm AT, T-38 tin foil coffin
Next battle will be Viipuri. RPs are awarded quite generously in this dlc and I have about 2,000 RPs now. Naturally I use only elite replacements. Gives your troops just the extra strength and resilience they need and besides, there are more than enough for any hardware you want.