I beat most of them against painful AIs. Many of them I had to really turtle--just my nexus and the two nearby nodes. Wall off the entrance with guns and stay put until I could field juggernauts. The map where you have a bunch of holes in the wall around your base I couldn't beat on painful and on many of them I didn't even try some of the starts as they were obviously going to get sandwiched.
There are still a couple of initial nodes that are slightly too far away but I found an alternate approach--a constructor can take such a node by building a gun rather than capturing. You get the capture anyway and the gun takes care of the guardians that pop up. You need the gun anyway to defend against orbitals. A pair of constructors can even take a moderately defended node this way.