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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-blam!-ing arsehole. We knew about this too and weren't sharing so it wouldn't get patched. I mean, the room is easy enough without a spawn delay but it makes it a little faster to get through. Way to go man, people like you are why we can't have nice things...