At first this seems very simple, but it is a lot of fun!
Hey, just soloed it as a defender!
Okay, here is how you beat it alone:
1) Chose the defender - in the late game fighting off the spiders is the limiting factor
2) Check all the lines and find out which are the shortest and longest
3) Focus on keeping the longest ones running, these need to finish first. Fix them, fight off siders attacking them, only fight off spiders on the two shortest when you do find the time and DO NOT fix the shortest line
4) When the three longest lines have turned green, clear the shorter lines of spiders and start fixing the second longest. When pressure from spiders is high, kill all spiders in sight, fix one section, fight again and so on. Do not worry about taking some damage (as long as you don't fall below 1/2, do always immediately kill large spiders that attack you and bombarding spiders only if they do hit you but keep repairing when they don't) and don't stop fixing when a single spider attacks your line, you will be able to kill it after finishing fixing. And always waste your last shots to reload if after fighting you are low on ammo!
5) At last fix the shortest line. This should be short enough that you dont' have to run much to reach spiders and ideally when you were able to fix it, you can cover everything on one screen
It still requires some skill but by now I can consistently beat it