Wow, sorry I was playing DMC3 that time so I immediately thought that you talked about it
Mission 18 is the mission you get Sparda right?
When you get it you enter on some sort of hole and after that in a red painting. Inside it you get to swim and after obtaining some free orbs you fight some Sins with your Needlegun. After you beat them a door opens and you get to walk again. Then you arrive in a room who has that sort of cycle that activates when you hit it multiple times. Do that, and you'll end up swimming again. When that happens you must swim as higher as you can, and when the water falls you must find a place to remain high. When you do in the highest place there is Philosopher's Egg. Take it, head down through the stairway and enter a red door. Inside you use the Egg in a Goblet which flashes and Nightmare appears. Beat him and take the Philosopher's Stone who was made while you were fighting and Bingo!