This can be done solo or with a friend (easier with a friend). This strategy requires the use of artillery/long range units to hold B and C. Build up a wall of artillery/ long range unit (Kodiak, Blisterback, Locust, etc.) between B and C. From this position most enemies will be spawn killed as they enter the line of sight of your units. I was able to make it to round 50 using this strategy in co-op.
Here is a video guide with English commentary!
Here is a playlist of guides for this game!
https://www.youtube.com/playlist?list=PL-HeW6XkKdUIfVlB38c4h...