I'll have to try this again, but during our first couple runs with FC mates, on the second boss, I tried to kill the slimes over the ruptured gas leaks but nothing happened. Was it only a certain time that it'll work? This was why after trying multiple times to kill slimes over the leak, I gave up and thought it was an incorrect method. Or maybe killing one slime over the leak wasn't enough. Further confirmation is much appreciated.