Just go walk to where the bombs will be, the second you see the "Break the Chain Reaction" warning. Stand next to it as it spawns, and then immediately hold the button. The only reason I've missed a bomb is if I'm dead.
That said, some people are saying they get two. I have no idea how they're doing that.
It's not always necessary to get two anyway. Like the text says "Break The Chain Reaction" - you die if too many bombs near you are still connected to each other in one group (I believe this is why some people survive while others die - if you and both of your neighbours do their job, but their neighbours don't, you may survive, but they may die).
It used to be possible to survive by going behind the solid (non-water) pillars at the edges of the room. I haven't tested if this is still the case, but if it works, then at least you can have survivors other than the Tank to do pickups. The downside to that being, of course, that there's fewer people to do bombs. Not recommended, but if you're in one of those frustrating groups that absolutely won't do the bombs, at least you may be able to finish the fight with just a few savvy Players this way. You'll also know who you can likely rely on to be situationally aware for the flooded room section, which is no small thing.