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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It's not really a cheese or glitch, just how the aggro and spawning works. Wave 1 can do it too a bit. The enemy spawns are linked together, so as long as you don't aggro the thrall by the right side the linked spawns won't appear. As long as the last mine to spawn is middle you can have minimal trouble.