It is possible to kill this boss without either part of it dying (by stunning it using the 'Choco/Mog' spell). Likewise, if his purple half is destroyed first, the Lost Number will use more mid-level spells, like Quake 2, against your party. If you blow off his red half (the one that uses magic) his physical attacks become more aggressive, so keep your HP up. Rarely, he does a double-fisted hammer punch to one character that averages 2200 points of damage, though, so pack in some Phoenix Downs before trying to tackle him. Aside from physical attacks, all this guy can really do is use a Bolt 2 spell on you, but it only does 400-500 points of damage to one character, so it's nothing to worry about. Heal yourself and use Summon Materia (like Choco/Mog) and you'll win in no time. Magic spells (Fire, Ice, Lighting) that are at level 2 are also okay to use. This boss can be taken out just by attacking and using Limit Breaks. When the safe opens, a Summon Materia (Odin) pops out, and you are assaulted by a big goon (how'd he fit into that little safe?) Don't forget to press O to verify each entry. This means that you hold right and scroll through the numbers, then hold left until you reach 10, then hold right for the last two numbers. Right to 36, Left to 10, Right to 59, Right to 97 Furthermore, unless you're fast enough, you'll run out of time if you try any other combination.
What most people forget when trying to open the safe is that it's just like a real combination lock.so you have to go in more than one direction when inputting the numbers.