Vincent’s Ultimate Weapon, Death Penalty can be obtained from Lucrecia’s cave.Head to this cave with Vincent in your party and a scene will commence. After the cutscene, engage in a few random battles and return to the cave. Press [OK] where Lucrecia was first standing and you will get both his final limit break and the Death Penalty. Lucrecia’s Cave can only be reached using a submarine or a gold Chocobo.
For information on how to obtain a blue/green or black Chocobo or to navigate to the cave using the submarine, please go to the walkthrough section. Vincent’s weapon will do its max damage depending on how many enemies you have killed. Vincent’s weapon can also make use of the overflow glitch which exists.
To do a one shot kill on any boss or enemy, you must reach around 65,000 Kills (this will take days). Alternatively, equip Vincent with Deathblow, and have him drink 5 or 6 Hero Drinks (as well as max his stats to full – see Stat Maxing guide in walkthrough) and have him kill at least 16000 enemies or so. The exact formula for how this damage is calculated is as follows:
[Total Attack Power * [Vincent’s kills Ã· 128 ] / 16 ] – source (Steve Huijboom). I don’t know the exact accuracy as I haven’t really tried it but it seems to work for what I have attempted with it. The only boss that can’t be one shotted is Ruby Weapon (as far as I know).