Well, this is another irony. I'm still in Stage 4, but I'm now late zone 4. I've been doing EVERY map on challenge assault just to see if I can do this without any upgrades.... And I got all the way to The Un-World before I found a map that I genuinely could not figure out how to beat in the time required. (it also did not help that the way its designed with the narrow halls means that portions of the map were just spent turtling.) The main reason I ran out of time is due to that Screaming node in the exact center of the map. There were eight, EIGHT hexes left undiscovered on that map. I cannot believe how close I was. The problem is one of those hid a shield tower preventing me from even hoping to accomplish this.
The problem for this map is that (I don't know why the shield nodes aren't showing on the screenshot above), but that big block in the center is protected by a shield node. If you are on challenge mode the turrets have a max range of 5. The thing that was setting me back so much was that screaming node in the center. That screaming node is 6 tiles away from most of the upper and lower paths you could build on. And the ones that are within 5 still means it'd take 50 shots to take it down.
I didn't have enough time remaining for such a strategy the first time I did this. So that was out the window. I went through the last 3 maps on landmark instead of challenge assault. So I could finish stage 4. Now I'm waiting on the 50B PL to start stage 5 :/