Taken from Apathetic Aardvark's FFVII Guide

Head to the Shinra Mansion in Nibelheim.

Go up the stairs to the second floor. Take a left when you get up them. Enter
the room to the north end of this screen. There is a safe. I warn you now, if
you have not saved in a while, you may want to, there is a boss inside and he
can really give a beating. You will have twenty seconds to open the safe.


The combination is
Right - 36
Left - 10
Right - 59
Right - 97

If you overshoot a number, your combination will fail. Upon opening it you are
going to be attacked by...

Name: Lost Number [Boss]
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Level: 35
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Information
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HP: 7000
MP: 300
Atk: 80, 80, 140
Matk: 35, 55, 5
Def: 120, 40, 400
MDef: 280, 440, 200
Def%: 1
Dex: 82
Luck: 1
EXP: 2000
AP: 80
Gil: 2000
Win: Cosmo Memory
Steal: -
Morph: -

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Elemental Resistance
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Fire: -
Ice: -
Lightning: -
Earth: -
Poison: -
Gravity: Nullifies
Water: -
Wind: -
Holy: -

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Status Effect Resistance
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Death: Immune
Sleep: Immune
Poison: -
Confusion: Immune
Silence: Immune
Darkness: -
Fury: -
Sadness: -
Stop: -
Slow: -
Frog: Immune
Small: Immune
Slow-Numb: Immune
Petrify: Immune
Death-Sentence: Immune
Manipulate: Immune
Berserk: Immune
Paralyze: -
Seizure: -
Positive: -

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Preparing
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Where to find: Shinra Mansion
Appears with: -

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Attacks
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1st Form: Punch [1 Enemy] [341], Bolt2 [1 Enemy or All Enemies] [525]
[Lightning]

2nd Form: Tentacle [1 Enemy] [852.5], Aspil [1 Enemy] [ 40.5]
[Absorbs MP], Bolt [1 Enemy or All Enemies] [270] [Lightning], Bolt2
[1 Enemy or All Enemies] [675] [Lightning], Quake [1 Enemy or All
Enemies] [371.25] [Earth] Quake2 [1 Enemy or All Enemies] [810]
[Earth], Bio [1 Enemy or All Enemies] [337.5] [Poison-Element]
[Poison-Status], Bio2 [1 Enemy or All Enemies] [708.75] [Poison
-Element] Poison-Status]

3rd Form: Punch [1 Enemy] [905], Lost Blow [1 Enemy] [2828.125]

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Notes
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Tips/Tricks: If a Magical Attack brings him below 3498 HP, he will
split into Form 2. If it is a Physical Attack which does it, he will
split into form three. He can not change forms if afflicted by any
status which stops his ATB bar. There is also a chance it will take
one to three additional hits to cause the split. His stats also change
depending on which mode he goes into. Lost Blow will likely kill you
at this stage of the game, even in the back row and a barrier on.

He hits hard and often. The best strategy is to get the 'stop' status on him
using any method you can. Cross Slash, Choco/mog, added effect=time. Any of
these will make the battle a lot easier. When he is stopped, just blast away
at him. If you elect not to do this, when he is just below half life, he will
rip one of his two colors left. The Red one will be a caster and will be very
resistant to magic. The Purple one will attack like you will not believe and
even your best attacks will do little damage. If he is stopped while he goes
below half life, he will not split until he can move again.

Once you have beaten him, grab the Odin Materia on the ground. Also, reach
INTO the safe to get the key to the basement. Once you have all of this leave
the room and head right. Keep heading right until you can not. Head up and
get the chest in the room for a Magic Source. Head down and enter the hidden
stairwell. Go down the stairs. Run down and exit this screen. On the next
screen rather than going left and into the lab area again, head up and use the
key you just got on the door. Open the casket in the middle of the room. Tell
him about Sephiroth. He will close his casket, open it again and ask who he
is. Once the chatting is done, exit this room and try to leave the basement,
Vincent will now join you.

Remember, those are not my own words. Credit goes to Apathetic Aardvark.

taiwanesealn