I earned this by playing level 13 on Practice Mode on Easy Difficulty.
I placed 6 gun towers in the 6 tower spots in the big U that leads up to the core housing.
I upgraded these to level 3.
Around wave 16, the dropship aliens come in. So I used the command ship to raise the buttress up to the core housing.
I then put a gun tower on each of the inner spots and upgraded these to level 3. I did this with the Scatter Shot Augmentation at level 3, so you may need a few more towers, or I'd recommend using General Cai.
I ended with 40k resources left, so realistically you should just be able to spam the towers in every square. Note that upgrades tend to be better than more towers, however, so I would upgrade what you have first.
This should unlock at the score screen.
Edit: I apparently made a video that I never posted here. This shows my final tower placement.
Downvoters please comment.