I think the lite version is a good indicator for how the rest of the game plays out. For level 4 I think I focused on engineers in the beginning to secure the middle, top left and bottom left rigs. They need to ride the jeeps to get there fast. Then protect those groups with one sniper each. Then you can advance to the right, securing the other 2 groups of rigs. At that point you can take the enemy base, or simply starve them of oil.
I wouldn't say the game was too difficult, just that it may take a couple of iterations to figure out how one is supposed to do it. I guess that's what I meant by "linear" in my earlier post.
Later in the game I felt that the missions required more aggressive tactics, and if you tried to play it safe you would end up getting overwhelmed.
Hope that helps!