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.