I just started working on the solo campaign. Although I don't have the level names, the first two levels were difficult for me on "normal".
TSAR Tank- 1st level. You need to have four Level 2 artillery pounders located at the 4 pads farthest away from your toy box that overlook the field from where the assaults come from. And all most all of the small pads down the land need to be occupied by L2 mortars. Why? Because your pounders will get the health of the tank down to about 50% and once it hits the lane, your pounders won't be able to shoot it any more. This includes a couple of large pads on the lane, the two-three closest pads to the toy box are worthless as artillery because they don't swivel far enough to the right. And this is why you need mortars pounding it. I barely got it in time. One of the first things you have to figure out is what is the swivel range of placed artillery. Some maps have large pads but the location and swivel range makes them next to worthless.
The second level is an air assault level where you start out with only one AA gun and 5 trifolkers launch and try to bomb the heck out of it. The really bad thing is that because it sits out in front and is exposed, there is also enemy artillery hitting it. After 3 failed attempts to defend this unit with constant repairs (that add up), I decided at the very beginning of the fight to sell it and move it farther back towards my toybox. This made the level relatively easy especially when you get the AA upgraded. I placed artillery where the AA used to be and killed the enemy artillery. The rest of the level was fairly easy.