It is the hardest of the French campaign for me, I only got one star. Slogging through the hordes of Russian cavalry at Smolensk took me 15 turns. They were absolutely spamming them until I could finally get an opportunity to take the stable. Im not eager to replay it and try to go for 3 stars.
The tactic I used was sending all 3 of my artillery units (Napoleon, Kutuzov, and Jourdan) South with Gates on infantry and Davout north with the rest of my generals they meet up at Smolensk and then encounter the spam. Took forever to get through.
I built lots of artillery to clear the many Russian units before Smolensk. I upgraded/built the industrial centers at the start for the much needed industry. When I captured Minsk I built a factory near there to spawn artillery closer to the battle. Once the Russian units were clear I rushed Moscow to end the mission. I managed this using the 3 free generals from US campaign and Kutuzov
If you want to avoid the spam, catch it early. Instead of helping Murat and Poland, go right up the middle with all of your general's and units and take the objective by force. Then say "Russia, I'm coming for you" and charge right into Smolensk and take that stable. Notice the angle that my artillery is at. Try and dip around the side so you can hit the stable easier. The enemy only got 3 units out before I shut that down. The Russians will have 3 generals where Murat was but a line infantry per round will hold them off and probably kill all 3 of them. Watch out for Bagration in the south. If you can spare one turn then hit him with Napoleon and get him out of Poland's hair. After taking Smolensk go right for Moscow. I was attacking it on round 12.
The things I did at the start:
Upgrade capital to level 5. Build Rocket artillery for Jourdan. Deploy Kutuzov on the existing rocket (If there is one. I might be tripping) Go immediately for the first objective.