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