Well the main thing is to secure your ugrade and spawn points and don't let AI units cross the gunners spot. All points should be secured by few units - as you can see 3 spawn points are protected by crabs (there should be around 4 per each spot). They are slow, but also hard to defeat.
Next thing is setting spawn points of basic units right into gunners spot. Gunners will block most of AI units (if some of it cross the gunners, crabs should end job). You can set spawn point by clicking on basic units spot and moving to your choosen location. Well now you just need to wait 2-3 hours and both achievement mentioned in guide should pop up :) Watch out - achievement appears only after won game so if you loose it - it won't count.

Could you explain this setup?

I have yet to play the game but I am curious after looking at your guide.

I would like to know what's happening here.