Go to Nibelhiem. Enter the mansion to the north. You can look at the letter to the far left to get some hints about the combination. Go up both sets of stairs and go left. Go into the room on Cloud’s right. There is a safe here. Examine it and pick the top option. You now have 20 seconds to input the safe combination. You must hit the number exactly and input it with O. You can’t go past the number by a few digits then go back again input it as this won’t work. The combination is: 36, 10, 59, 97. If you put it in correctly, the safe will open and you will be attacked…
Just attack with your best spells and summons – Choco/Mog works well as it usually paralyses this boss. You need to be careful after you inflict enough damage as the boss will shed of it’s parts, leaving it either red or purple. If it is red, magic won’t hurt it as much, so use physical attacks. In it’s red form it will primarily use magic attacks on you. If it is purple, physical attacks won’t work as well, so use magic attacks. In purple form it can deliver some nasty physical attacks so keep your HP up.
When you have beaten the boss, pick up the red Summon Odin materia, Red XIII’s Level 4 Limit Break manual, Cosmo Memory, and the Key to the Basement. Go to the left from the top of the stairs and through the door in the corner of the lower room. Go down the spiral staircase and go south to the second screen (before the library). You can now open the door to the north. Examine the closed coffin and talk to Vincent.
Tell him about Sephiroth and name him. Keep talking to him until he tells you to leave him alone. Attempt to leave up the stairs and Vincent will come out and choose to join you.