Gedo can bring forth lightning. Katsuyu is composed of 70-85% water. Meaning fire will evaporate her insides and lightning will be conducted by all the water in her, thus killing her as the membrane concludes being dry.Her one and only attack is enough to kill Orochimaru. She is enormous compared to any other known summon. That's enough. Also the fact that Katsuyu can't be killed by physical attacks. No summon possesses enough firepower to take such a huge entity down. Her moving around is enough to crush the other summons.