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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	Eh Smart move is save a cursed thrall in round 1 and 2 at the end for picking up ammo and revivies synth timers etc and to run around clearing mines Also have a person croutched not moving at each round start on the far ends and usually only a few thrall pop and youve got a solid chance of being under a mine at round start to save time