Zhitomir mission is reasonably difficult, completed in 16 turns. I found a stout defense on the Malin-Fastov line worked very well. Key thing to remember is don't get seduced by the IS-1, keep using those cheap infantry in good terrain backed by ZiS-3 or SU-85 or cheapo ATG to fight tanks. A B4 is also helpful for raining pain down on the enemy. There's an excellent blob of forest to anchor your middle with a solid pack of infantry + ATG. IS-1 is so expensive to reinforce that I regret upgrading two of my KV-1 to it, and should have instead kept with them for the infantry support role while having an independent IS-1 btn to provide heavier power to the armor.
This is not an attacking scenario so getting aggressive is not advised. There's tons of PaKs in them there woods and the AI can concentrate an enormous amount of armor hidden all over the place to hammer you if it wants to.
You get something like 26 CP of reinforcements on land and 6 CP of reinforcements in air, so keep that in mind when determining initial allocation of forces.
Keeping the partisans up north alive during the initial attack by retreating them north into the woods and evacing whatever other kit you can helps greatly. They can clean up retreating Germans.
The aux M17 was very nice and I hope I unlock the ability to purchase them soon... it'd be nice having useful lend lease kit rather than junk. The other nice unit is the M9 halftrack which I've got on my heavy infantry. I would especially like to have it on my engineers since they're my primary tank support unit but they only get trucks!
Zhitomir has a bug where there are 5 listed objectives west of Kiev, but only 4 displayed on the map. I lost only one of them, and retaking it had me at 4/5 until I reloaded at which point I magically got 5/5.
Sorry about the giant pictures, if there are spoiler tags I'm happy to use them. I don't see them in the editor and [spoiler][/spoiler] doesn't work.