are you making liberal use of artillery cannons? a quantity of them can replace a single defense node
when i played it out, i think i was running low on time when i fought my way into the core
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 :/