Here's something interesting that I had come up in the run that I just submitted to the co-op leader board (link goes directly to the time in question)

At 1:07:57, the underground grabby hands of the boss don't grab me at all, meaning that I can fire away unharmed during that attack pattern. When this first happened, I assumed it was because of the specific spot that I was standing in, but on second watch it seems like I may be completely immune prior to standing in that location. My suspicion is that this only works in multiplayer when one person dies. I tried standing high on the platform in single player story mode and it didnt work. I also tried standing high on the platform when both of us were alive and it didn't work. This seems to only apply to this specific one-person-is-dead scenario, which is a bit weird. It makes me wonder if there are other things you might be immune to if one person is dead and if it only works if you are on the screen where they died, as would be the case in boss fights.