I figured this out with a clan-mate first time we attempted this. 
We accidentally triggered a delayed spawn, and we wanted to know how we did it, so we kept experimenting till we found the cheese! 
Basically what the cheese is:
Round 2 and 3 have a delayed hive spawn, so you're able to finish off the bombs before the wizards and boomers come out to annihilate you. 
https://www.youtube.com/watch?v=iy5vm668VwA
						
					
					
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	Its hard but fun,we wiped there like 15+ times? We could never get the mines down in time. But thats what made it fun